tree: Remove redundant local_hist variable, use tree.GetHistogram()
context.py: Add locks for some thread safety
Reorder invisible_canvas to hold new locks for minimal length of time
context: reset gPad to None if it was none before, add gDirectory assertion
context: add new context managers useful for TTree draw and other things
tree drawing: don't pollute gDirectory, multithreading robustness enhancements
rootpy browse bugfix: dont try to delete browser if it is already closed
rootpy browse: avoid key duplication
context: don't check for 'is None' because it's different from PyROOT_NoneType
context: contextmanager *must* be instantiated each time.
interactive.wait: update canvases
rootpy browse: Ensure canvases update when commands are run
Merge pull request #185 from pwaller/tree-fixes

TTree draw, context and rootpy browse enhancements