Merge remote-tracking branch 'origin/master'

Readme.md

 Heroku buildpack: Python
 ========================
 
-This is a [Heroku buildpack](http://devcenter.heroku.com/articles/buildpacks) for Python apps.
-It uses [virtualenv](http://www.virtualenv.org/) and [pip](http://www.pip-installer.org/).
+This is a [Heroku buildpack](http://devcenter.heroku.com/articles/buildpacks) for Python apps, powered by [pip](http://www.pip-installer.org/).
 
 [![Build Status](https://secure.travis-ci.org/heroku/heroku-buildpack-python.png?branch=master)](http://travis-ci.org/heroku/heroku-buildpack-python)
 
@@ -18,14 +17,13 @@ Example usage:
 
     $ git push heroku master
     ...
-    -----> Heroku receiving push
-    -----> Fetching custom build pack... done
+    -----> Fetching custom git buildpack... done
     -----> Python app detected
-    -----> Preparing virtualenv version 1.6.4
-           New python executable in ./bin/python
-           Installing setuptools............done.
-           Installing pip...............done.
-    -----> Installing dependencies using pip version 1.0.2
+    -----> No runtime.txt provided; assuming python-2.7.3.
+    -----> Preparing Python runtime (python-2.7.3)
+    -----> Installing Distribute (0.6.34)
+    -----> Installing Pip (1.2.1)
+    -----> Installing dependencies using Pip (1.2.1)
            Downloading/unpacking Flask==0.7.2 (from -r requirements.txt (line 1))
            Downloading/unpacking Werkzeug>=0.6.1 (from Flask==0.7.2->-r requirements.txt (line 1))
            Downloading/unpacking Jinja2>=2.4 (from Flask==0.7.2->-r requirements.txt (line 1))
@@ -37,13 +35,20 @@ You can also add it to upcoming builds of an existing application:
 
     $ heroku config:add BUILDPACK_URL=git://github.com/heroku/heroku-buildpack-python.git
 
-The buildpack will detect your app as Python if it has the file `requirements.txt` in the root. It will detect your app as Python/Django if there is an additional `settings.py` in a project subdirectory.
+The buildpack will detect your app as Python if it has the file `requirements.txt` in the root. 
 
-It will use virtualenv and pip to install your dependencies, vendoring a copy of the Python runtime into your slug.  The `bin/`, `include/` and `lib/` directories will be cached between builds to allow for faster pip install time.
+It will use Pip to install your dependencies, vendoring a copy of the Python runtime into your slug. 
 
-Hacking
--------
+Specify a Runtime
+-----------------
 
-To use this buildpack, fork it on Github.  Push up changes to your fork, then create a test app with `--buildpack <your-github-url>` and push to it.
+You can also provide arbitrary releases Python with a `runtime.txt` file.
 
-To change the vendored virtualenv, unpack the desired version to the `src/` folder, and update the virtualenv() function in `bin/compile` to prepend the virtualenv module directory to the path. The virtualenv release vendors its own versions of pip and setuptools.
+    $ cat runtime.txt
+    python-3.3.0
+    
+Runtime options include:
+
+- python-2.7.3
+- python-3.3.0
+- pypy-1.9 (experimental)