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.

it always uses HTTP Basic Auth, whereas our endpoint is publicly
available.

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