Provides name and message per error case

[tkalfigo]

Major 1) Passes specific error name when creating instance of generic JwtParseError class.
Useful to end user to know precise error type and not just the message so he
can take additional action depending on it.
Major 2) create() respects user provided 'exp' timestamp in claims param over the default of one hour from now.
Minor: Indentation/formatting.

[gyszalai]

@robertjd Please merge this...

[typerandom]

@tkalfigo We're very thankful for this PR. But unfortunately it's quite old (over a year) and contains too many changes to just merge in.

Instead I've created a few new issues that consolidate the changes that were seen in this PR:

• Code should follow Crockford's JavaScript formatting conventions. See: Formatting should follow Crockford's JavaScript formatting conventions #21.
• Errors should be identifiable and documented. See: Errors should be separately typed and documented #22.
• The create()-method should respect user provided exp timestamp in claims over the default one 1 hour one. See: create() should respect exp timestamp in claims #23.