Add a default Rake task that does nothing.
Add sample RSpec integration spec.
Run specs as part of default rake task.
Add simple integration spec using Rack::Test.
Git ignore tmp directory.
Move location of log file into its own directory.
Rubygems is included automatically in recent versions of Ruby.
Prevent real HTTP requests when running specs.
Add some specs around the Harmonia assignments action.

Note that I made some changes to the application code in this commit in order
to make it easier to test:

* I converted a bunch of settings which were being stored in local variables
into proper Sinatra settings so they could be accessed from the specs.
* I changed the "dictionary" of Harmonia people names vs Trello member IDs to
instantiate `Trello::Member` objects.
* I moved the `card.update!` statement inside the conditional where the due
date is set, because I realised that `Trello::Card#remove_member` and
`Trello::Card#add_member` do not rely on this call.
Use same authentication mechanism for Harmonia & Trello requests.

* Reject Trello event requests without authentication token using "410 Gone"
so old Trello webhooks are deleted [1].
* Add a few specs for Trello events actions.
* Use string literals for Trello events path - extracting into a setting was a
step too far.
* Make local variable names more consistent.

[1]: https://trello.com/docs/gettingstarted/webhooks.html#deleting-webhooks