Update JWT.php

Add a method to retrieve the header. OpenIdConnect tends to add a "kid" to the header that is used to determine which key to use for verification. Getting header information allows you to figure out which key to pass to "decode". Other possible ways to handle this, allow $key passed to decode to be an array of keys. For now, let's just let people get to the header.
Update JWT.php

Try to add the RS256 algorithm.
Update JWT.php
1. Allow encode to take a "kid" parameter
2. Allow decode to take an array of keys, and use the "kid" to look up the key.
3. Add RS256 as an algorithm
4. Add tests for the above
Updates for some whitespace and other minor issues.
Merge branch 'master' of git://github.com/BambooHR/php-jwt into BambooHR-master
whitespace cleanup
Merge branch 'master' into BambooHR-master
Merge pull request #11 from firebase/BambooHR-master

Bamboo hr master