Add a blank line and comment the per() aliasing

I was editing the per() function and my editor decided to re-indent
the line that aliases the method so it was *inside* the method. This led
to great hilarity and confusion. Hopefully adding the blank line and
comment will help avoid this problem.
Support .per(value,unit) and similar as in doc

Specifically, the .per(), .plus(), .minus(), .times(), and .isSame()
methods all support either a single Quantity argument, or a pair of
(value,unit) arguments which are converted to a Quantity on the fly as
shown in the README example.

Also fixed syntax error in that README example, added example of a
predefined Quantity, added test cases for new features, and removed
the 'u' shortcut in the test cases - it isn't really useful now that
the various methods can create a Quantity on the fly. (This way the
test cases are written more like you would expect actual code to
be written.)