Moves key utilities into a separate gem.

relaxed gem lock level

Address github bug 39

fix github issue bitpay#40

Fix GitHub issue 40

Version 2.4.0 adds refund convenience methods:

* Allows creating refunds for invoices
* Retrieving all refunds on an invoice
* Retrieving a specific refund
* Cancelling a specific refund

correct BTC validation

bumped gem version

Rename the Client class to SDK::Client

This is a breaking change. We apologize for not bumping
the major version, but there are business reasons that is locked.

removes the BITPAY_PEM environment variable and any code that uses it.