Add download_python function to reduce repetition

Create a common download_python() function that allows most other files in the build/runtimes directory to be simplified to something of the form:

```
#!/usr/bin/env bash
# Build Path: /app/.heroku/python/
# Build Deps: libraries/sqlite

OUT_PREFIX=$1

source ./download_python
download_python "2.7.8"
cd src

./configure --prefix=$OUT_PREFIX
make
make install
```

builds/runtimes/download_python

+#!/usr/bin/env bash
+# Build Path: /app/.heroku/python/
+# Build Deps: libraries/sqlite
+
+# download_python(PYTHON_VERSION)
+# Example usage: download_python "2.7.8"
+download_python()
+{
+    PYTHON_VERSION=$1
+
+    if [ -z $PYTHON_VERSION ]
+    then
+        $PYTHON_VERSION="2.7.8"
+    fi
+
+    echo "Building Python $PYTHON_VERSION..."
+    SOURCE_TARBALL="http://python.org/ftp/python/$PYTHON_VERSION/Python-$PYTHON_VERSION.tgz"
+    curl -L $SOURCE_TARBALL | tar xz
+    mv Python-$PYTHON_VERSION src
+}