(wip) Add inline bucket support.

![status](http://img.shields.io/status/work-in-progress.png?color=red)

This commit adds support for writing small buckets directly inline to their value in their parent's leaf node. Previously, subbuckets would simply have a bucket header stored in their parent bucket which pointed to the root page. This required that every bucket use at least a single page. This has a high overhead for buckets with only one or two small items.

Inline buckets checks subbuckets to see if they only have a small amount of data (about 1kb) and no subbuckets. If these conditions are met then the bucket's root node is written to a fake page which is simply a pointer to the end of the bucket's header.

Fixes #124.