Remove useless Catch case.

While I'm here, I might as well.
Simplify TimeParser.

To simplify the codebase a bit, TimeParser is now:

* No longer constructed with a customisable unit map; the units are mapped to
  multipliers by an internal if statement.
* No longer a template parameterised over the types involved; the output type
  is now fixed to an alias of `std::uint64_t`.
Add TimeParser test.

This is not a complete test suite, as it does not check exceptions.  Currently,
I'm unsure how to use Catch for exception tests, so we only check the 'valid'
cases.

The RHS data has `ull` suffixes due to overflows when previous iterations of
the unit tests were run on Travis.
Fix typo in TimeParser test.
Unify position typedefs.

AudioSource::MicrosecondPosition, PlayerPosition::Unit, and
TimeParser::Time are all now TimeParser::MicrosecondPosition.
Remove long-form units.

As per discussion with @LordAro.
Clean up Multiplier.

Fix Doxygen comments, and use our new-found horizontal space to make the
Multiplier members have decipherable names.
Merge pull request #44 from UniversityRadioYork/mw-time-parser

Simplify TimeParser. (attempt 2)