Refactors impurity functions as cost functions, and improves performance.
Cleans up tree-building code.
Fixes infinite recursion bug.
Changes splitting function for slight performance improvement.
Makes tree-building function compute complexity.
Adds prediction functions for new data.
Changes classtree to use refclass trees.
Fully transitions to reference class based trees.
Move tree2.R to tree.R and update documentation.
Adds collapse point calculations and node removal.
Removes old code that is no longer used.
Reorganizes Tree class for clarity.
Fixes copy-constructor error for ClassTrees.
Adds predict function for ClassTree.
Adds cross-validation framework.
Fixes roundoff errors with ClassTree pruning.
Adds tentative support for cross-validated ClassTree.
Changes makeTree's signature to be more like rpart.
Small fixes and documentation cleanup.
Merges branch 'develop' for v0.1.

This version includes a function for building classification trees as described
by the CART algorithm, as well as a separate function for performing k-fold
cross-validations.