Compatibility with non-rvm switchers

Specifying a gemset in ruby-version is not compatible with switchers
(chruby, rbenv) that do not have the intrinsic concepts of gemsets.

  See http://stackoverflow.com/a/15709012/36378
Merge pull request #133 from halostatue/ruby-version-compatibility

Compatibility with non-rvm switchers
Resolve #126: Exception annotation.

This implements Raven.annotate_exception as described in #126, capturing
the annotation as ``@__raven_context``, which is treated the same as the
options parameter to Raven.capture_exception.
Merge pull request #134 from halostatue/exception-annotation

Resolve #126: Exception annotation.