Add a framework for libraries, and a first library gc

- Libraries are under src/lib/
- Added to build.js as usual, before plugins.
- See src/lib/README.md for details

gc library implements a "garbage collector" library, which allows
both the core and plugins to store elements and listeners to a list,
and when impress().lib.gc.teardown() is called, to have all of them
removed from the DOM. It also allows plugins to register their own
callback functions, which are called at teardown.

Commentary:

This work is based on copying the src/lib/gc.js from impressionist. While it was
useful, it turns out on the impress.js side there was much more a need to reset
attributes rather than delete elements. For now, this means lots of plugins do this
via their own lib.gc.addCallback() functions. Probably it would be nicer to add
some generic lib.gc.resetAttributes() functionality for this particular case.
I'll return to this in a future patch.

extras/ are not supported for impress().tear(). What can I say, they're extras.
Maybe in the future I'll support them, for now I can live without.

build.js

@@ -2,6 +2,9 @@ var buildify = require('buildify');
 
 buildify()
   .load('src/impress.js')
+  // Libraries from src/lib
+  .concat(['src/lib/gc.js'])
+  // Plugins from src/plugins
   .concat(['src/plugins/navigation/navigation.js',
            'src/plugins/resize/resize.js'])
   .save('js/impress.js');