Enh/tkregister2

Adding interface for Freesurfer tkregister2 command, so far only with basic functionality to convert fsl-style registration matrix to freesurfer-style matrix
These doctests are failing:
```
TraitError: The 'moving_image' trait of a Tkregister2InputSpec instance must be an existing file name, but a value of 'func.nii.gz' <type 'str'> was specified.
```

This is happening because there isn't a file called ```func.nii.gz``` in the test data directory (```nipype/testing/data/```). You can either set an existing filename (probably best) or create the name issuing ```touch func.nii.gz``` in that folder and adding the new file to the repository.

Same stuff for ```func2anat.mat```

Other than that, this contribution is nice. Thanks a lot.