Use importlib instead of __import__
BF: Add backport of importlib for python 2.6
Merge pull request #1 from arokem/stefanv-importlib

ENH: Backport importlib
BF: Relative import
BF: Absolute import instead.
BF: Remove the import that was not in the try clause.
Merge pull request #2 from arokem/stefanv-importlib

BF: Relative import
Merge pull request #471 from stefanv/importlib

Use importlib instead of __import__
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.
BF: Set the zero-d_sig case to have np.eye(3) evecs.

Also, deal with setting of min_signal. This still needs a little bit more
cleanup.
TST: One more test-case; relative signal.