Bug where (tree != tree) returned incorrect results
Added a utils module. Added reduce(), count() methods in STree
STree is now picklable even with parents. Added tests for STree.

"Bug": Pickle removes parents from STree, so calc_parents is required on
load. Currently a test is failing, and a fix is planned for the future.
find_predicate is now deprecated. Use filter instead
Fixed a bug where repeated use of selector-lists gave wrong results
Marked as version 0.4.3