Added new magnitudes package, related docs, and tests.

Added new `magnitudes` package. This handles basic conversion between magnitude and flux, and would be very useful for `pysynphot`. Also added related docs and tests. Updated existing docs where necessary, including a minor typo in `Quantity` docstring. My Emacs is also set up to auto-delete trailing whitespace.

p/s: This is based on idea from @eteq (see attached pic).

![flux_conv_tollerud_131008](https://f.cloud.github.com/assets/2090236/1328700/b46890ea-3507-11e3-9eab-8524ae92f363.jpg)