Added BUILDOZER_P4A_DIR env var behaviour
Made p4a distribution dir name == app.package.name
Minor code cleanup (formatting)
Added env var querying for default and bool
Added env var checking to all config get methods.

The new method replaces the manual env var parsing with simply setting
the config's value to the env var value. This lets the normal parsing
methods do the work later, to avoid duplication of effort.
Added env check during config build
Added a few comments and formatting changes
Added app.p4a_dir token
Variable renaming for clarity
Merge pull request #60 from inclement/p4a

Add ability to choose python-for-android directory
implement the `clean` command.
Merge branch 'buildozer_clean' of http://github.com/kivy/buildozer into buildozer_clean
rename `clean` to `distclean`