Add option to adjust fill percentage.

This commit adds the ability to adjust the fill percentage for splitting nodes. This
works by setting a threshold that is a percentage of a total page size. When that
threshold is crossed during a split then a new node is created.

This is primarily beneficial for append-only workloads.

Fixes #163.
Change verbiage, fix node test.
Merge pull request #166 from benbjohnson/fill-percent

Add option to adjust fill percentage.