Converted test/unit tests to RSpec specs.

Test -> spec translation maintained the same functionality/test coverage.
Removed specs from gem distribution since they currently ping a live RTM
account, requiring some test-specific setup.
Removed forced 1 second sleep in RTM API calls.

RTM has ok'd burst traffic at higher rates, so milk_cap is now leaving any
necessary API throttling to applications using this library.
Corrected Task#add! to use same RTM timeline as other API calls.

RTM timelines can be reused across multiple changes to RTM data.  This removes
an extra API call to create a new timeline for each #add! call.
Added support to set task tags.

Task objects now support a tags= method to set the object's tags.  Task#save!
will update the tags based on current value w/ RTM.