Add typed error handling

[typerandom]

Fixes so that all errors are typed and documented.

How to verify

Read the docs, checkout this branch and test it with the code below:

var nJwt = require('njwt');

try {
    nJwt.create();
} catch (err) {
    if (err instanceof nJwt.SigningKeyRequiredJwtError) {
        return console.log('Signing key required!');
    }
    console.log('Unhandled error!', err);
}

Fixes #22

[coveralls]

Coverage remained the same at 100.0% when pulling 5904561 on fix-error-handling into 9e177c3 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 60cb403 on fix-error-handling into 9e177c3 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 60cb403 on fix-error-handling into 9e177c3 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1fd51b4 on fix-error-handling into 9e177c3 on master.