fix(#160) : changed AuthHttpError to Error

[HNeukermans]

Investigation of issue #160:
I found out, that, currently Typescript does not support deriving from Error class, or at least has difficulty doing that. Extending from Error doesn't work when emitting ES5
Therefore, I changed AuthHttp to throw an instance of class Error instead of class HttpAuthError.

This fixes issue #160.

[HNeukermans]

Token feature:
Currently, JWTHelper class is only way of asking information about the token.
Therefore, I created a separate Token class. This class is a statefull implementation of JWTHelper methods, and responsible for providing information about the token.

Updated the implementation.

1. Token constructor, now throws exception if an invalid value is passed in.
2. I removed the Token.Empty implementation since it has become useless.
3. Added IPayload interface that exposes an 'expiration' API.
4. I made Token to have a decodePayload() method, which returns an IPayload instance.

FYI: commits 2 & 3 don't belong to fix(#160), but make part of a separate 'feature' commit. My apologies.

[chenkie]

Thanks for your PR :) I had to merge some others in first. Can you rebase so I can merge this?

[paulogr]

Any news about this issue?