Fix FieldDistribution to count relative to specified tree

Previously, counts were record-based however this effectively "broke"
the distribution for fields that went through a many-to-many
relationship.

The caveat to this fix is that fields that contain values that are not
associated to the records in the root model will not be included in the
distribution (with a count of 0).
Merge pull request #61 from cbmi/issue-60

Fix FieldDistribution to count relative to specified tree