Merge pull request #181 from pwaller/rootpy-browse-ipython

$ rootpy browse: Drop into an ipython shell with names imported from file
add File to registry
Merge pull request #182 from ndawe/master

add File to registry
minor edit in rootpy browse script options
Merge pull request #183 from ndawe/master

minor edit in rootpy browse script options
ability to switch off snake_case alias
Merge pull request #184 from ndawe/master

ability to switch off snake_case aliases
use upper case for negations
Merge pull request #186 from ndawe/master

use upper case for negations in rootpy script
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.