Ignore vim swap files.

I was using vim for this branch, and notices that these aren't being ignored.
Raise non-fatal error when an invalid seek occurs.

Currently, this just throws a WHAT at the user.  This should be
fixed, along with the other WHATs, before milestone 0.1.0.

Fixes #17.
Fix #33.

This makes playd throw a WHAT when a non-numeric seek is found.
Internally, a SeekError is thrown.
End file on sox-based seek failures.

Or, in layman's terms, if you seek after the end of a file, it'll
behave like you'd expect.
Merge pull request #34 from UniversityRadioYork/mw-seek-rescue

Seek Rescue! (tm)