Ensuring working tree passed into feature writer.

As previously written FeatureWriter maintained a reference to WorkingTree
that was never initialized. Causing a NPE on calls to remove() and next().
Merge pull request #314 from jdeolive/fw_wtree2

Ensuring working tree passed into feature writer.
Merge commit 'a625f1ebd10dfffa940b1e913a69698faa561278' into SprintInProgress