Reworked unit tests for better test coverage.

[geofffranks]

Now uses Test::Fake::HTTPD to verify that the data input into WebService::Pushover is what we want to get sent to the remote server. As a side-effect, this also gives us testing of the API contract that WebService::Pushover provides to end-users.

This required some changes to Pushover.json, since the documentation for Net::HTTP::Spore wasn't good enough to determine how to change per-method base_url values for testing.

[hakamadare]

fixed a dependency in Build.PL that was making Travis unhappy