Added example for 2D-histogram with variable bin size.
Fixed two typos.
MAINT: Remove uses of the WarningManager class.

WarningManager was a workaround for the lack of the with statement
in Python versions < 2.6. As those versions are no longer supported
it can be removed.

Deprecation notes are added to WarningManager and WarningMessage, but
to avoid a cascade of messages in third party apps, no warnings are
raised at this time, that can be done later.

Closes #3519.
MAINT: Fix Python version dependent test.

The buffer test in numpy/core/tests/test_api.py was apparently
testing for incorrect behavior that was corrected in Python 2.7.5.
This PR changes the test to check for the correct results and makes
running it depend on Python version.

Closes #3512.
Merge pull request #3522 from charris/fix-test_api

MAINT: Fix Python version dependent test.
Update twodim_base.py
Merge pull request #3520 from charris/replace-warningmanager

Replace use of WarningManager by warnings.catch_warnings and then deprecate it.
Update twodim_base.py
Merge pull request #3531 from fkbreitl/master

Updated documentation for histogram2d() in twodim_base.py according to discussion