readme update
When palette menu item is clicked, trigger itemdropped event and invoke dropHandler (to be implemented)
Merge branch 'master' of https://github.com/marclitchfield/VisualProgramming
Restructured expression tree as binary tree.

prop, sub, and call are now top-level expressions.
Started work on the parser, which prompted the expression tree
restructuring.
isAssignable logic is dramatically different, and now includes terminal
checking rather than having it be a separate constraint.
Tests have been refactored and are now much simpler, and no longer require
modififying expression trees that have already been materialized.
Enable parser tests that currently pass