Add JEP 6 proposal
Update JEP to allow "_" as a valid unquoted identifier
Add note about compliance test differences
Implement JEP 6
Add unicode literal compliance test
Update spec with the JEP 6 grammar

Also removed the grammar from the README and replaced with a link.
Must people looking at the README don't care about the actual
grammar (other than to know that there is one).  For those that
are interested, they can follow the link to get to the grammar.
Also avoids us having to keep the grammar in two places.  Now the
official grammar lives in docs/specification.rst.
Remove duplicate test case
Merge branch 'jep-6' into develop

* jep-6:
  Remove duplicate test case
  Update spec with the JEP 6 grammar
  Add unicode literal compliance test
  Implement JEP 6
  Add note about compliance test differences
  Update JEP to allow "_" as a valid unquoted identifier
  Add JEP 6 proposal