Add upgrading doc.
Update changes with "_" instance reference.
Rename _Underscore to _Dash.
Add note about new features in upgrading section.
Bump dev version.
Add callback arg inspection support for classes that implement __call__.
Remove unnecessary callback default.
Add numerical module.

Closes #31
Fix test for round_() so that it passes in both Python 2 and 3.
Add median().

Refs #31
Add docstrings for numerical functions.

Refs #31
Add mapiter().
Add numerical module to docs.
Release v2.1.0.