Rename grn_token to grn_token_cursor

Because it's not "token" itself. It generates tokens. We're using
"cursor" for object that returns the next ID by "_next()" function.

So grn_token_cursor name is better for the current grn_token object.
token-filter: use token filter specific callbacks instead of generic callbacks

Generic callbacks are difficult to understand how to use API because
its signature doesn't show what arguments are passed.

Specific callbacks are easy to understand how to use API because its
signature shows what arguments are passed.
Merge pull request #214 from groonga/improve-token-filter-api

Improve token filter API