Fixed some formatting issues with mpc.Observation when using 
comments and null observations.
MPCWriter now uses mpc.Observation.to_string to include comments 
on the same line without separate calls to write_comment.
ProcessRealsController just uses MPCWriter.write_mpc_line method
instead of write_comment and write_rejection_line.
Removed write_comment and write_rejection_line methods from 
MPCWriter.
Renamed write_mpc_line to write.
Merge pull request #164 from drusk/mpc_reject_format

Closes #161 and closes #162.