Used Gridplot to provides subplots support to mpl. Implementation 2x2-specific, we need to generalize.
Added basic functionality to deal with the grid geometry. Bonus: code templated.
Used add_plot session method instead of the old plotcontext.children.append, after land of #427 on master.
Added subplot example.
Fixed some unused imports and comments.
Fixed the example to render labels at the yaxis.
Deleted rendered examples.
Fixed missing datetime import.
Merge pull request #458 from damianavila/fix_dt

Fixed missing datetime import.
PyPy compatibility: esentially avoid pandas-related functionality and skipping some other test because NumPyPy does not pickle arrays yet.
Added default False to is_pypy.
Fixed the previous logic. It only checked for pandas-related stuff if pandas was installed, forgetting about the numpy possibilities.
Simplified logic using two main if flows, one for pandas objects and the other one for numpy objects.
Update to sync master.
Commented and unused pandas import.
Added skip decorators to downsample tests. Also capturing import error because the lack of SciPy.
Little refactoring adding some @Matheuz's suggestions.
Deleted unused commented code.
Don't duplicate session logic for each type of data store
Conform to Redis' behaviour in {InMemory,Shelve}Session

mget() returns None on non-existent elements, so use
dict.get(key, None). srem() doesn't fail on non-existent
elements, so use try: set.remove(key) except KeyError: pass.