Add Groonga::DoubleArrayTrie#update

This is a low level API for `grn_table_update_by_id()` and
`grn_table_update()`.

[GitHub #22] Bind `grn_table_update_by_id()` and `grn_table_update()`

TODO: add an easy-to-use API (e.g.: Groonga::Record#rename).
Use my name
Add missing "static"
Remove needless `NIL_P` check

Because `rb_grn_scan_options()` accepts `nil` as `options`.
Unindent
Merge pull request #24 from myokoym/bind-grn-table-update

Add Groonga::DoubleArrayTrie#update

Patch by Masafumi Yokoyama. Thanks!!!