ENH: Adding '.' as an na_value for FRED.
Adding some tests to cover the FRED data fetching code.

This a very minimal, but it wasn't previously covered.
I've added a new to contain the tests and more or less copied code
from the test_yahoo file.
ENH: Better error message if a bad series name is passed to the
get_data_fred function.
DOC: Updated RELEASE.rst with change notes.
Merge pull request #3469 from TomAugspurger/fred_na

ENH: Adding '.' as an na_value for FRED.