Port tests to rspec

[tristanpenman]

This branch ports the few existing unit tests to rspec, and adds several tests for the Client class, leveraging the mock support provided by Savon (the SOAP Client gem).

The tests for the Client class could be more complete, but the tests that are there serve as a starting point for building a more comprehensive test suite.

[tristanpenman]

Note that rspec tests can be run using the following command:

bundle exec rake spec

And code coverage reports will be written to the coverage directory.

[shawr]

looks good