Create sglfit function

[jstriaukas]

Translate the sglfit function from R:

https://github.com/jstriaukas/midasml/blob/master/R/sglfit.R

[jstriaukas]

add corresponding: tscv.sglfit, cv.sglfit and ic.sglfit functions.

[jstriaukas]

fixed the Fortran link to Python - sglfitF.f90 can be compiled on several systems (tested on Linux and Windows 10).

[jstriaukas]

added sglfit function at:
https://github.com/jstriaukas/midasmlpy/blob/master/midasmlpy/sglfit.py

also added a description of how to compile Fortran code and about the files which are precompiled.

[jstriaukas]

added tscv_sglfit function at:
https://github.com/jstriaukas/midasmlpy/blob/master/midasmlpy/tscv_sglfit.py