Add JWT setters

[typerandom]

Adds JWT helper functions for setting custom header and body keys.

Important

Since we've done a major refactor (see #31), this is based directly on that refactor. So before we review this, #31 needs to be merged.

How to verify

Tests have been added that verify the functionality of these new helper methods.

Fixes #25

[coveralls]

Coverage remained the same at 100.0% when pulling 38d53cf on add-jwt-setters into 9e177c3 on master.