Remove unused fmt pkg import
Rename package to nexmo

Having 'go' in the name is not needed, dropping it in favor of just
using 'nexmo' makes the usage clearer and more concise.
Use enviroment variables to store testing credentials

Instead of forcing users to edit the file and include their API
credentials, where they could potentially end up checked in to their git
repository and/or end up posted on Github, use enviroment variables to
store them.

They can be set globally by exporting them, then calling the program, or
you can pass them to `go test`:

NEXMO_KEY=something NEXMO_SECRET=something NEXMO_NUM=18005555555 go test
Update README.md

Add a godoc button, update examples
Wrap lines at 80 characters

Improves readability, especially in terminal editors and on Github.
Remove erroneous comment
Cleanup sendMessage()

Convert is_flash_message to camel case
Simplify declaration of all the string arguments
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