Make token format compatible with Esprima and Mozilla-styled locations.

* Tokens are now instances of single exported constructor Token.
* Token objects are compatible with Esprima (only `type` format is different).
* Added token.loc in format of node.loc (Mozilla).
* Deprecated token.startLoc & token.endLoc.
* Updated comment generation example.

Also added ability to pass arrays in `onToken`/`onComment` where
tokens/comments will be collected in Esprima's format so you can
simply pass those arrays to `escodegen.attachComments`.

Updated docs and comment attachment example.