Improve readability of meta language

Some ideas for improvement:

[x] All reserved rules should start with .
[x] All assignments to properties should use :
[x] String references should start with _
[x] The "separated_by" rule could take two arguments (by_rule rule) instead of (by_rule){rule}
[x] Add support for simple single line (//) and multiple line comments (/* .. */)
[x] Use name instead of "name" in the rule names, disallow spaces
[x] name:"alias" instead of @"name""alias"
[x] Prevent node and node references from starting with _
[x] Prevent node and node references from starting with .
[x] "token":"property" instead of "token""property"
[x] Use = for assigning a rule to a node

PistonDevelopers / meta