Adds _Record.affected_start and .affected_end.

These coordinates should represent the zero-based, half-open region of
the reference sequence affected by all the events included in ALT. These
coordinates allow the user to identify precisely which bases are altered
by the events in the record.

Provides more thorough documentation on the coordinate schemes for
_Record.POS, .start, and .end.
Merge pull request #161 from gotgenes/feature-affected_coordinates

Adds _Record.affected_start and .affected_end.