Adds an optional date argument to the `log` command that can be
specified just before the task alias.
Some examples from @skoppelmanCC:
hcl log yesterday @documentation +2 Ran spellcheck
hcl log 3 days ago @wireframes +3.5
hcl log last friday @admin +1 filed timesheets
Commands#log:
Copy over the contents of `Commands#start` and replace `task.start` with
`task.add`. This allows us to log the time without actually starting a
timer (and thus having to subsequently stop that timer).
command_test.rb(#test_start):
Update expected method call arguments with `spent_at`. Now that we're
passing in `spent_at`, the expected arguments should include it.
command_test.rb(#test_log_failure):
Delete this test because I've rewritten the `Commands#log` method to no
longer use timers.
utility_test.rb: Add tests for `#get_date`
Fixes #83.