Reorganize functions.R and disperse functions over multiple files

[dustinvtran]

It's preferable to have multiple functions that are roughly of the same category belonging to the same file (or directory). One file should not collect everything however.

[dustinvtran]

There should also be separate folders for different AISGD implementations, e.g., one for SVMs and one for GLMs. Auxiliary functions that are useful for all implementations should stay in the root directory, others that apply specifically to a model should be put into that folder.