Fixed Powerline "distutils-r1" phase issues.

The Powerline ebuild now respects "distutils-r1" phase mandates by overriding
python_*() rather than src_*() phase functions. (A number of other minor
improvements abound, as well.)
Fail if no USE flags are enabled.
Added USE flag "fonts".

This commit comes largely thanks to pull request #2 (ZyX-I), which, due to
recent changes, necessitated manual resolution. Cheers, ZyX.
Added "app-vim/powerline-python".

Such package now provides Powerline's Vim plugin, a change necessitated by the
proscription of conditionally inheriting eclasses.
Added fishman's version of Exuberant Ctags.

As of late 2013, fishman's github clone of Exuberant Ctags is the most actively
maintained version. This ebuild cleanly installs that, along with Gentoo's suite
of patches applicable to the official 5.8 release from 2009.
Reverted accidentally committed AGS regression.
Added jargs 1.0.

Such Java package is a hard dependency of YUI compressor 2.4.8 (to be
subsequently added). Since Portage fails to provide such package, add such
package locally to simplify user installation of YUI compressor.
Added YUI Compressor 2.4.8.