Fixed trial period marshalling issue, added test

If you try to subscribe a customer to a plan with a trial period, Stripe will throw an error. That is because `stripe-haskell` just uses the `Show` instance of `UTCTime` which generates a string like `"2014-11-13 02:29:01.694179 UTC"`.  Stripe expects a numeric timestamp instead of a rendered string.

This pull request:
* bumps the version to 0.1.1.0 due to the slight type incompatibility of `newtype TrialPeriod`
* adds a test for the broken case.  You can view a branch with this test but the old, code which exposes the issue here:  https://github.com/docmunch/stripe-haskell/tree/mc01-fix-trial-period-old-version-failing-test
* makes a note in COVERAGE.md