Adding bspline fits readers

Don't merge until [#71](https://github.com/astropy/specutils/pull/71) is merged.
Figured out how to process the spline coefficients in FITS. Created a file which has dummy spline coefficients. Created test cases for linear spline
@wkerzendorf Same implementation for the IRAF subclass as well. The error comes when [this](https://github.com/astropy/specutils/blob/master/specutils/models/BSplineModel.py#L39) line is called. The `cls` object belongs to `Spectrum1DIRAFBSplineWCS` and `return cls(degree, knots, coefficients)` tries to instantiate `Spectrum1DIRAFBSplineWCS` instead of `BSplineModel`, and I get `TypeError: __init__() takes at least 6 arguments (4 given)`. I am not sure how to fix this with the current architecture. Any ideas?