Rename NexmoWithApiAndKey

Changed to New()
Use build tags for tests

To test sending messages, use go test -tags messages
This prevents users from sending text messages accidentally when testing
Provide more useful error output on message tests
Add NEXMO_FROM env var

Allow user to set the 'from' value when running tests
Limit API requests to 1 per second
Add TODO for rate limiting
Add apiRoot constant
More descriptive test messages
Add check for client reference length
Rename New to NewFromAPI()
AccountBalance can be a function-local type
Create Account type
Start splitting code up logically
Add USSD support, split in to multiple files
* development: (21 commits)
  Add USSD support, split in to multiple files
  Start splitting code up logically
  Create Account type
  AccountBalance can be a function-local type
  Rename New to NewFromAPI()
  Add check for client reference length
  More descriptive test messages
  Add apiRoot constant
  Add TODO for rate limiting
  Limit API requests to 1 per second
  Add NEXMO_FROM env var
  Provide more useful error output on message tests
  Use build tags for tests
  Rename NexmoWithApiAndKey
  Cleanup sendMessage()
  Remove erroneous comment
  Wrap lines at 80 characters
  Update README.md
  Use enviroment variables to store testing credentials
  Rename package to nexmo
  ...
Merge branch 'master' of https://github.com/inhies/gonexmo
Updated comments and README to reflect recent changes.