MPC comment and reject formatting changes

MPC comments are now on the same line as the formal MPC record, separated by a space character (instead of using #).  Rejected sources have an '!' as their minor planet number.

The ```MPCWriter``` class now only has a ```write``` method (instead of ```write_mpc_line```, ```write_rejection_line```, write_comment).  The ```mpc.Observation``` object passed to ```write``` can simply have its comment property set in order to write a comment, and it can have its ```null_observation``` property set to True to have a rejection line written.

Closes #161 and closes #162.