TST - new DTI test to help develop min_signal handling
BF: If all the signal is zero, evals are 0. If all the signal is merely very
low, evals are very high.
BF - compute min_signal from whole data set.
RF: Deal with very small signal, and with all-zero signal.

These two cases should be separated: if s0 is non-zero, and all of the d_sig is
zero, the eigenvalues should be high (3.0 per default, the diffusion of water
in water at 37 deg C). If all of the signals are 0, we set the eigenvalues and
eigenvectors to 0.
Merge branch 'dti_min_signal_test' of https://github.com/MrBago/dipy into bago-dti-min

Conflicts:
	dipy/reconst/tests/test_dti.py