Document top-level .encode and .decode to close #459

[dimaqq]

Document jwt.encode and jwt.decode and close #459 (maybe, I mean, the classes need to be documented too, but maybe that's not as important).

Also clean up rst heading markup because shpinx complained about that.

[coveralls]

Pull Request Test Coverage Report for Build 195

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 178:	0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[jpadilla]

this is great, thanks!