Create 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
```
I would suggest using `local` scoped vars for PYTHON_VERSION.