Provide an error message if python isn't installed.

Closes #14.
Merge pull request #17 from nicksay/deps-python

Provide an error message if python isn't installed.
Upgrade to Closure Compiler v20140625 (requires Java 7).

Also update Makefile to output a status message when running Ninja.

Closes #16.

Simplify Ninja status message output in Makefile