Integer & float parsers, more aliases, and small improvements

FEATURE: sepBy2 combinator
FEATURE: apply function
FEATURE: integer parser
FEATURE: float parser
FEATURE: Parser#and() is an alias for append()
FEATURE: Parser#then() is an alias for sequence()
DOCS: Misc improvements and additions
BREAKING: removed some helper functions from the JSON parser
BREAKING nicer API for ParserAssertions