Edited tutorial to mention a few more methods of Quantity objects.

Added examples of using .dimensionality, .to_base_units, and
.ito_base_units.
Removed h alias for hour due to conflict with Planck's constant

Closes #82
Updated CHANGES
Add support for additional numeric types

decimal.Decimal and subclasses of numpy.number are now numeric types.
Make Quantity only accept numerics as magnitudes

When the input magnitude is not a numeric type or list/tuple that can be
converted to an ndarray, raise a TypeError.
Make Quantity add/sub return NotImplemented

The Python docs specify that when an overloaded operator is not
implemented for a particular type, the operator method should return
NotImplemented. This allows the interpreter to try fallback
operations (e.g. reversing the operation) to try and find another
method that will work.
Refactor Quantity mul, div, and pow operators

Create methods_imul_div and _mul_div to centralize the multiplication
and division operator methods. Also make multiplication, division, and
power methods return NotImplemented when appropriate.
Remove redundant magnitude computation
Switch to reStructuredText style docstrings