add loopless FBA implementation

Added to the flux_analysis submodule. This commit also includes unit
tests.
add loopless FBA documentation
Merge pull request #133 from aebrahim/loopless

Add loopless FBA implementation
cobra.io.mat split read code to from_mat_struct

The function to convert the struct read in by scipy.io.loadmat has been
moved to its own function called by load_matlab model