Added a testing file.  Still looking at different options for tests.  It's looking like py.test should be the way to go.
Added a small test.  More to come.
Another test.  More to come.
Unneeded comment.
Working on a more unified data structure for handling the unit stuff.
Removed unused arguments.
Fixed a weird bug that doesn't seem to have caused any errors!?  Still don't know why.  Need more tests.
Additional comments and cleanups.
Added a global debug switch to allow turning on pdb at a specific point in time (for interactive testing sessions).
Docstring for createMetricPrefixes.
FEATURE!  Allow the use of the right shift operator ">>" to perform conversions.

Normally you would have to write e.g. (20*cm).convert(inch) in order to have unit conversions performed.
With this new operator overloading for __rshift__, you can use the right shift operator to perform the
conversion.  The precedence appears to work correctly in the sense that all other arithmetic happens
first before the right shift triggers.  For interactive work, this results in several savings, including the
parens "(" and ")" which both require SHIFT, as well as typing out the word ".convert" and then the
parens for the convert function "(" and ")".

Here is an example session:

In [3]: from unity import *

In [4]: 10 >> 1
Out[4]: 5

In [5]: 10*cm >> cm
Out[5]: 10.0

In [6]: 10*cm >> inch
Out[6]: 3.9370078740157486

In [7]: 1*m >> inch
Out[7]: 39.37007874015748

In [8]: m > 39*inch
Out[8]: True

In [9]: m > 39.4*inch
Out[9]: False

In [10]: m > 39.3*inch
Out[10]: True

In [11]: 20*cm>>inch
Out[11]: 7.874015748031497

In [12]: 10*m+30*m >> inch
Out[12]: 1574.8031496062993

In [13]: 40*m >> inch
Out[13]: 1574.8031496062993

In [14]: 10*m + 30*m * 2 >> inch
Out[14]: 2755.9055118110236

In [15]: 70*m >> inch
Out[15]: 2755.9055118110236