RF: move construction of a simple HRF-convolution based dataset into data_generators

so we could reuse it in other tests.  Also enhanced it with our unused
autocorrelated_noise

Conflicts:
	mvpa2/misc/data_generators.py
make default noise in a simple_hrf_dataset to be a simple normal

autocorrelated noise requires more work since might produce quite funny result for long durations etc
ENH: improvements to simple_hrf_dataset

This is a partial cherry-pick from
d1babc67bbffdf7e4e5e2552a29e4b5a22dc8e58
in enh/glm_trial_estimates branch
ENH: simple_hrf_dataset -- can accept pre-crafted HRF array
ENH: use np.arange to create timeline for HRF + allow for 'naive' resampling instead of proper scipy.signal's

primarily for testing of HRF estimation based on FIR so we could seek identical
results
Merge pull request #219 from yarikoptic/enh/simple_hrf_dataset

RF code from test_stats_sp.py into simple_hrf_dataset so we could create "realistic" ones for testing