Escape special shell characters so that passwords with single quotes can
be saved to the keychain. Otherwise, the single quotes surrounding the
"-w '%s'" cause an error on passwords with single quotes.

I know testing private methods is not recommended, but this seemed like
something that would be good to test. Maybe the method should be made
public if that's a concern?

The first time I saw it, I wasn't sure what format of subdomain this
prompt wanted, and entered 'acme.harvestapp.com'. Since it's not
documented in the README either, I had to source-dive to find the
answer. Clarify what should be entered here by including an example in
the prompt.

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.