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.

By using pure faraday for the test and error handling code,
the way is cleared for dropping in alternative http clients.