Added a C implementation of MIM.
Moved the feature increment into the mex code, instead of in the C code.

Makes it easier to use the library from languages other than Matlab.
Updated Makefile with modifications from PyFeast.
Fixed stupid bug where the output of calloc wasn't checked.

Now uses the MIToolbox checkedCalloc function.
Fixed the feature increment, now calls code from MIToolbox.
Updated function definition to return pointer to selected features.

For compatibility with PyFeast.