Fixed Python runtime according to the official Heroku doc

Update Heroku Postgresql default add-on to hobby-dev

See https://devcenter.heroku.com/articles/python-runtimes#supported-runtimes

update shebang to use python2 for bpwatch

Make vendored libraries available at compile time (fixes #57)

Typo

When building, BUILD_DIR is set to a temporary directory. Vendored
libraries (e.g., pylibmc) are downloaded and unpacked in
$BUILD_DIR/.heroku/vendor/lib, but this is not currently in
LIBRARY_PATH or LD_LIBRARY_PATH.

Fix this by doing the following:

* Add $BUILD_DIR/.heroku/vendor/lib to LIBRARY_PATH/LD_LIBRARY_PATH

* Add $BUILD_DIR/.heroku/vendor/include to
  C_INCLUDE_PATH/CPLUS_INCLUDE_PATH

* Add $BUILD_DIR/.heroku/vendor/bin to PATH, so vendored commands can
  be used at compile time (e.g., curl-config)

* Add $BUILD_DIR/.heroku/vendor/lib/pkg-config to PKG_CONFIG_PATH, so
  vendored packages can be found with pkg-config

This reverts commit f78460e2.

Add Django v1.5 testing support with Heroku

Store changes made in post_compile in the cache.

Remove shared db from release and add heroku-postgresql dev db

Fixing the compile hook scripts paths

The compile script is run with the root of the git repo of the project
being pushed as the working directory.

$BIN_DIR is pointing to the bin directory of the buildpack which is not
where you would want to put the pre and post compile hooks.

Changing back to the old convention of looking for the hooks from the
bin directory at the root of the project.