Optimized NiftiMasker.inverse_transform()

A new function for unmasking has been added (unmask_optimized). It
is slightly less general than the previous one (unmask), but uses
three times less memory.
Generalized optimized unmask

Optimized unmask now works in arbitrary dimensions.
Removed previous unmask implementation, adapted tests.
Merge pull request #42 from pgervais/unmask

Unmask