Merge pull request #6 from jkakar/fix-hour-formatting

The time format correctly renders '1 h 00 min' when an activity took
exactly one hour.

Fixes #5.
- Update API documentation to define parameters and return values.
- Remove some dead code discovered in this process.
- Add a missing 'lib/timelog.rb' file and update requires in test
  files.
Merge pull request #7 from jkakar/yard-documentation

- The README now includes instructions that describe how to setup a
  development environment.

- A new `Rakefile` can be used to generate API documentation with Yard
  and to run the test suite.

- A missing `lib/timelog.rb` file has been added and imports in tests
  have been updated to use it.

- All modules, classes, etc. have been updated with descriptions of
  parameters, return types, etc. using Yard-compatible documentation
  syntax.