Refactor specs + fixes

- Cleanup + Refactor spec syntax (specs tend to be reused so why not
keep them well styled)
- Fixed Braintree a bit that had Test::Unit syntax in it, also
described pending tests
- Fixed SecurePayAU Gateway that had it active_merchant gateway
misspelled
- Fixed UsaEpay spec
- Add base classes for 11 gateways
- Updated read me and Versionfile
- Bumped dependencies
Remove braintree dependency

https://github.com/spree/spree_gateway/pull/138/files#r10213250
Add braintree as a development dependency
Depend on Spree 2.3.0.beta
Bump to 2.3.0.beta
Correct email + homepage + files for gem
Removed savon dependency

Fixes #139
Use new preferences setter in AuthorizeNetCIM class
Correct arguments for BraintreeGateway's capture method
Remove BraintreeGateway#preferences

This is no longer needed with the recent preferences changes on Spree's master branch
Use new preferences references within braintree_gateway_spec to appease the testing gods
Remove ancient pending block for authorize+capture payment flow within BraintreeGateway's tests
Ignored options on BraintreeGateway#capture should be optional
Use correct options when calling capture in BraintreeGateway#capture tests
Braintree Credit test won't work because Braintree doesn't provide a way (yet) to settle the transactions
Set auto_capture to true before void tests + call right void_transaction method
Call correct credit! method in BraintreeGateway's spec's credit_using_spree_interface method