Fixed handling of nullable plus parsers.

Need to test this more thoroughly before merging it in to 1.1, but it is
looking good so far.
Separated enhancements from new features in CHANGES.md
Merge branch 'nullable-plus' into v1.1
Fixed nullable plus bug
Updated change log with nullable plus bug fix
Merge branch 'abnf' of https://github.com/aengelberg/instaparse into abnf
Merge branch 'abnf' of https://github.com/aengelberg/instaparse into abnf
Working on abnf perf
Switched reductions to maps, rather than fns.
tweaking debug statements in gll
abnf tests
Used regexes to simplify handling of whitespace and comments
Smart merging of core rules
Added case-insensitive strings
Deleted re-escape, no longer needed
More flexible rule for ;

Now allow comments before and after ;
Added tag-hiding bracket notation to ABNF
Changed DEBUG back to false
Added abnf combinator implementation

Also improved ebnf combinator
Added abnf combinator to API