RF: cleanup hrf module with comments

We were duplicating finding of the numerical function, presumably
because I didn't properly understand what was going on.
NF: add SPM HRFs with tests

Make SPM-compatible HRF functions and test against results of SPM code
running in Octave.
TST: add tests against saved SPM basis functions

Use run from SPM code to test our SPM basis functions.
DOC: fill out docstrings and refactor SPM hrfs
BF: add hrfs mat file to data in setup.py

We weren't installing the hrfs.mat file.
RF: rename hrfs.mat file to spm_hrfs.mat
Merge pull request #298 from matthew-brett/hrfs-cleanup

MRG: clean HRF module and add SPM HRFs

Some small cleanup to hrf module.

This pull request adds the SPM HRF, the SPM temporal derivative, and the SPM spatial derivative, along with a function to replicate spm_hrf.m in the SPM distribution.

Add SPM HRF, with temporal derivative and spatial derivative. Check
against saved output from SPM code.