Added MustacheContextException

  Solving need to distinguish context errors from environment errors.
  The exception has fields for the offending key and line number. This
  is a first step towards internationalization of messages.
Changed MustacheContextException -> MustacheException.Context.

Also made data into final fields. I don't like unnecessary getters.
fixes #28
Merge pull request #29 from bruckner/master

Fixes issue 28, comments containing '}' parsed as text