Evaluation altorithms separated from misc

Created a new nipype.algorithms.eval group for methods performing
simple error, distance or similitude computations over images.

I think this is better than having them mixed up with, e.g.,
nifti or csv files management.

I also add a new interface: ErrorMap, that computes a distance
function between two images (e.g. two displacement fields). Several
metrics will be available, however now I contribute only with
squared differences maps.
Added masks support in ErrorMap interface
more informative message
added squeeze of 1 comp dimensions
Fixed error forming an extension of a filename
ProbtrackX2 interfaces, fixes #792
added a docstring
ran make specs, this changed several files, but did not generate a test.
Merge remote-tracking branch 'upstream/master' into probtrackx2
ran make specs after python install to generate auto tests
fixed example, added dummy files
added ProbTrackX2 to fsl __init__.py
modified CHANGES to include ENH FSL ProbTrackX2 interface
Fixing errors to attend #830
Minor changes to satisfy #830
Backwards compatibility in #830
Updated CHANGES
Including normalization in misc
Added test
Moved the new Similarity in 4D to metrics

Close #802. Please merge #830 before this PR