Added rms errors to list of constants

Took rms errors from Table 1 of the paper and added them as constants
(rms_err_BminusV, etc) in constants.py
set_constants now returns the rms_err as well
Function calculate_derivative_term is working

In order to calculate the uncertainty in the bolometric correction,
I need to calculate the derivative of the BC with respect to the color.
Since it's just a polynomial, I just needed to tweak the calculate_term
function from a_i * x^i to i * a_i * x^(i-1) in order to be able to call
it in a loop and calculate the derivative of the polynomial.
Adding in uncertainty

Things are getting messy, so it's time to walk away. In the morning, I
need to take a fresh look at how I'm handling the errors, and see if
there's a better, more streamlined way to go.
Finished implementing uncertainties

The code now handles uncertainties, and all tests are passing.
Updated docstrings to reflect new interfaces
Updated README.md
README.md more descriptive, fixed typos
Merge branch 'uncertainties' into develop