Remove unnecesary copy operation in truediv and floordiv

Close #80
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
Improved dimensionality checking in UnitRegistry.convert

As dimensionality for source and destination units are computer before
converting values, this patch compares them before to fail early.
Get get_dimensionality should work even for compound non-multiplicative units
Reindented code
Added an optional argument to get_base_units to return the scaling factor even if nonmult units.

The optional parameter allows collecting the scaling factor even if a
non multiplicative unit is found on the way.

Fix #85
Implemented is_multiplicative property in Definition and Converter. Removed delta_degK

- The OffsetConverter is multiplicative if the offset is 0.
- delta_degK is not needed because degK is multiplicative.
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.
Improved support for non-multiplicative units.

Quantities with non-multiplicative units are converted to base units
before multiplication and division.

See Issue #86
Removed 'R' alias for roentgen as it collides with molar_gas_constant

close #87
Make kelvin the default name for the unit and degK an alias

The plan is to deprecate degK for 0.5

See #86
Updated tests to use `kelvin` instead of degK. Add more tests.

See #86