Separated client

This merge request separates the bitpay ruby ruby client from the bitpay library. The bitpay-client gem is renamed simply 'bitpay'.
A follow up commit will create an additional project file with the bitpay-client gem, a simple command line client that uses the bitpay library with the file system to create a persistent interface with bitpay.com.

Also included in this merge request are additional argument checks on the currency and price inputs to the create invoice method. 

Finally, this merge request contains cucumber tests that cover the library specification.
Done.