User-specified output ratio

Added a keyword argument which enables the user to select other ratios for the output image rather than
just 1:1.
Corrected indentation and spacing.
Corrected indents (4space instead tab).
Corrected indents Again.
Added usage example.
Refined syntax.
Removed blank lines in between parameters.
Renamed parameter
Syntax and default variable.

From (0,0) to None.
Adapted if statement to grid_shape=None
Added test case for grid_shape parameter.
Resolved faulty montage2d import.
Merge pull request #603 from TheChymera/master

ENH: User-specified output aspect ratio in skimage.util.montage.montage2d function