Delete this test because I've rewritten the `Commands#log` method to no
longer use timers.

I had added the 'date' argument because I wanted it for the `log`
command, which delegates to `#start`, but now have put that directly in
`#log`. As such, it's no longer needed here and the method can be
reverted back to what it was before.

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).

Still need to work out how to eliminate repetition between this and the
`#start` method, but this is now functioning for logging time to any
date (previously it was only possible to `log` to today or yesterday,
otherwise a timer would start).

Document this new argument in the help.

Before I modified it, the code passed `Date.today` plain in `spent_at`.
Don't bother `strftime`ing since it can just take a Ruby date object.

Now that we're passing in `spent_at`, the expected arguments should
include it.

If `#index` doesn't find a match, it will return `nil`. Correctly handle
this case in `#get_date`.

Since the date can be `nil`, it's easier to do the `Chronic` parsing in
`#get_date` than in `#start`. Finally, move the `#strftime` to `#add` to
allow it to handle `nil` dates at the source.

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

Fixes #83.