Signer and Base64URL decoding fixes

1. Includes fix for Base64URL decoding padding #9 (thanks @alexaubry!)
2. Key based Signers can now be initialized with unencoded keys:
   public init(key: String)
   The existing String based initializer was modified to highlight the fact that the key is expected to be encoded:
   public init(encodedKey key: String, encoding: Encoding = Base64Encoding()) throws

0.4.1: Improve initializing and signing

Small API improvements to 0.4.0

Ready for Linux

• fix tests for Linux
• use Node instead of JSON for payload
• represent dates as Ints instead of Doubles
• flip logic of NotBeforeClaim and ExpirationClaim

I heard you like protocols

This release features a redesigned API that allows customizing many aspects of the creation of tokens using protocols for signing, encoding, and data storage.
Also included is a playground with examples on how to use VaporJWT.

APNS Compatibility

• added base64URL encoding option (for Apple Push Notification Service compatibility)
• use correct algorithm for P-256

Actually useable (?)

• Fixes import and exposes API publicly
• Allow specifying fully custom header

Sneak preview

First release. It does not yet support verifying the payload nor does it support all encryption methods but hopefully it is useful to someone :)