Base commit splitting the dti utilities

A new preprocess file will contain all workflows regarding preprocessing
of EPI images.
Renamed susceptibility pipeline

Since I'm to replace this workflow, I rename it with its ``classic''
name. The new pipeline will be based on the latest versions of
```fsl_prepare``` and ```epi_reg``` of FSL.

Additionally, my intention is to provide with an interface for
the new tool ```topup``` and a new workflow for T2-to-B0 registration-based
correction.
Extracted EPI-specific interfaces from utils & dti

Created a new file for adding EPI-related interfaces.
Base files for new epi interfaces

Added base classes for interfaces fsl_prepare_fieldmap, topup and applytopup
Prepare interface implemented

Start testing
Finished fsl_prepare_fieldmap interface
Added interface for topup
Fixed details in topup

Still needed to check outputs
Added interface for applytopup
Fixed ApplyTOPUP and TOPUP
Added workflow using fsl 5.0 for correction

Susceptibility correction with fieldmap
Finished and tested the new fieldmap_correction wf

Additionally: now fsl.PrepareFieldmap gives a 4D output, as it
is requested by fugue.
Added support to set the encoding direction
Added automatic generation of encfile in TOPUP
Finished workflow for TOPUP

Also, part of the ApplyTOPUP has been refined
Updated changelog

Before proceeding to submit the PR.

Close nipy/nipype#661
Merge branch 'master' into enh/NewPreprocessDMRIWorkflows

Conflicts:
	nipype/interfaces/fsl/utils.py
	nipype/workflows/dmri/fsl/dti.py
Changed n. bins
Merge branch 'bug/FixAntsInterface' into enh/NewPreprocessDMRIWorkflows
Added some useful outputs