Add tests for coverage

Some coverage is low because CLDR will be used in production. Once this has been isolated, the coverage will be elevated.


```
[13:13][apipkin:~/Projects/intl-messageformat-polyfill(tests)]$ npm test

> intl-messageformat@0.0.1 pretest /Users/apipkin/Projects/intl-messageformat-polyfill
> grunt jshint

Running "jshint:all" (jshint) task
>> 2 files lint free.

Done, without errors.

> intl-messageformat@0.0.1 test /Users/apipkin/Projects/intl-messageformat-polyfill
> istanbul cover -- _mocha tests/ --reporter spec



  message resolvedOptions
    ✓ empty options

  message creation
    ✓ simple string formatting
    ✓ simple object formatting
    ✓ simple object with post processing tokens
    ✓ complex object formatter
    ✓ complex object formatter with invalid valueName
    ✓ complex object formatter with offset
    ✓ Simple string formatter using a custom formatter for a token
    ✓ Simple string formatter using an inline formatter for a token
    ✓ Simple string formatter using a nonexistent formatter for a token


  10 passing (7ms)

=============================================================================
Writing coverage object [/Users/apipkin/Projects/intl-messageformat-polyfill/coverage/coverage.json]
Writing coverage reports at [/Users/apipkin/Projects/intl-messageformat-polyfill/coverage]
=============================================================================

=============================== Coverage summary ===============================
Statements   : 94.05% ( 79/84 )
Branches     : 81.82% ( 54/66 )
Functions    : 100% ( 8/8 )
Lines        : 94.05% ( 79/84 )
================================================================================
```