XUnit output formatter

[ap4y]

Added xunit output formatter and ability to change formatter for the rake task via formatter environment variable. xunit formatter based on code by @sebastianludwig as a part of tuneup.js.

[listrophy]

Currently requested list of formatters:

• colorful: already implemented; default
• passthru: don't do anything to instruments' output, just pass it through
• simple: strip out most info
• xunit: per this PR

Some/All of these will be implemented in the next release, which will be based on the discussion in #21.

[listrophy]

Please see my comment on #42 about either having this output xunit xml directly to stdout or creating a new class of plugins to deal with this kind of thing. Looking forward to your input.

Thanks!