Merge branch 'release-0.2.1' into develop

* release-0.2.1:
  Bump version to 0.2.1
Add copyright to license file

Fixes #36
Update grammar with new filter expressions
Update perftest to measure lexing time
Allow a single compliance test to be run via env vars
Add support for syntax error tests
Properly support AST repr with unicode chars
Initial support for filter expressions
Add syntax error tests for filters

This also pulls in the syntax.json file from the
jmespath.php repo with a few small modifications.
Don't allow literals as child element in subexprs
docs: fix error in grammar
Support literals with numbers
Add more compliance tests for literals

Fixes an issue with escaping the escape char.
Update specification with new grammar
Use non empty JSON objects
Initial commit of jep7
Fixing syntax.json

Using non-empty arrays in objects in the compliance test for better cross-language support for things like PHP and Lua.
Add single keys to given hash types
Merge branch 'mtdowling-jep-7' into jep-7

* mtdowling-jep-7:
  Add single keys to given hash types
  Fixing syntax.json
Update specs with filter/literal exprs