Drop support for Ruby 1.8
Remove deprecated :json option and dependency on faux gem
Remove existing oauth functionality and dependency on oauth gem
Tweak version of dependency on webmock gem to eliminate addressable warnings

Webmock 1.18.0 depends on addressable >= 2.3.6, which includes this fix:

https://github.com/sporkmonger/addressable/pull/146
Remove constructor block functionality
Eliminate positional args in favour of :key and :secret options
Use JSON.generate instead of JSON.dump
Eliminate Nexmo::Response class in favour of raising exceptions
Remove dependency on mocha gem
Minor code tweaks for clarity
Use application/x-www-form-urlencoded requests instead of application/json

cf. https://github.com/timcraft/nexmo/issues/24
Add Nexmo::AuthenticationError exception for 401 responses
Add methods for sending USSD messages
Add methods for sending US Pre-approved Short Code messages
Add methods for Voice API
Update README
Add note about environment variables
Merge branch 'v2'