Brain, Model, and Nifti object inits should support named arguments

[jeremymanning]

The following should work:

• std_bo = se.Brain('std') <-- calls se.Brain(se.load('std'))
• std_model = se.Model('std') <-- calls se.Model(se.load('std')); since the standard brain has only one volume, this is a silly example (the model is a matrix of all zeros)
• std_nii = se.Nifti('std') <-- calls se.Nifti(se.load('std'))

(and same with other named files and/or when the user passes in a filename for a .nii, .nii.gz, .bo, or .mo file)