Add custom keyword hooks (for #79)
Document new methods
Merge branch 'master' into custom-validation

Conflicts:
	test/all_concat.js
	test/all_concat.js.map
	tv4.min.js
Added limits on custom errors, clarified README
Ban duplicate codes as well as code-strings
Merge pull request #111 from geraintluff/custom-validation

Hook for custom keywords