0.2 release

This is just a tracking PR for changes in an 0.2 release.

If you have changes that are not minor bugfixes then do your PR against this branch (0.2-wip).

So far:

 * I've bundled the `batch()` optimisations brach where it creates a `WriteBatch` in the V8 thread rather than queueing up operations and making it in the async thread.
 * I've merged in @juliangruber's CBatch stuff to have a play with. No commitment on it actually making it in but I'd like to see what kind of performance characteristics it has when it's done up nicely with the full range of error checking & `Buffer` handling.

My current thinking is that if we do go with a chained batch here in LevelDOWN we should put it into `batch()` (like someone suggested for LevelUP way back, perhaps it was @juliangruber?). If `batch()` gets no arguments (or no callback argument at leas) then it returns a `Batch` object that can be used to build and commit, otherwise, if you feed it an array then it'll do what it currently does. So we'd support both styles in the same method.