ENH: N4BiasFieldCorrection new arguments

Now, it is possible to save the estimated bias field, by fully supporting
the --output argument specification.
added metadata to input trait in N4BiasFieldCorrection

Both comments satisfied (https://github.com/nipy/nipype/pull/911#discussion_r16982315)
Merge pull request #911 from oesteban/enh/N4-output-bias

ENH: N4BiasFieldCorrection new arguments