sourcemaps v3 generation

Added generation of v3 sourcemaps ( serialized as single base64 mime encoded url, with original sources embedded). Tested with Chrome 31.0.1613.0 canary.
Line numbers are tracked same way as with --line-numbers or --firebug style debug info generation. Ideally I'd like to use start/end line/column from lexer down to compiler/parser, but even with just line numbers it's already very useful. 

See related issues: https://github.com/LearnBoost/stylus/issues/1072 https://github.com/LearnBoost/stylus/pull/886

CSS sourcemaps in Chrome [howto](https://developers.google.com/chrome-developer-tools/docs/css-preprocessors#toc-enabling-css-source-maps)

Screenshots:

![screen shot 2013-09-02 at 3 30 21 pm](https://f.cloud.github.com/assets/173025/1065525/ee59d026-1391-11e3-8972-368e15efb02c.png)

![screen shot 2013-09-02 at 3 30 38 pm](https://f.cloud.github.com/assets/173025/1065527/f2280592-1391-11e3-82dd-ca433c01ede3.png)