* Moving `DecodeSegement` to `Parser`
This would allow us to remove some global variables and move them to parser options as well as potentially introduce interfaces for json and b64 encoding/decoding to replace the std lib, if someone wanted to do that for performance reasons.
We keep the functions exported because of explicit user demand.
* Sign/Verify does take the decoded form now
This will allow clients to pass, for example, their own instances of
rsa.PublicKey if the key is not specified as some flavour of X509
cert. For example, Salesforce just specify the modulus and
exponent (https://login.salesforce.com/id/keys)
Renamed type SigningMethodRS256 to SigningMethodRSA
Added contstants SigningMethodRS256, SigningMethodRS384, and SigningMethodRS512 to support each of these methods
Added simple tests to support these new methods