Switch to using spam, not Matrix, for sparse matrices

This seems to be much faster, if somewhat harder to deal with
Add spam to Travis binary install list
Fix sub_score_mat() to deal with 'spam' sparse matrices
Add function to add in pairwise scores to a sparse score matrix
Fix subtle bug when querying 1 against 1 from spam sparse score matrix

spam doesn't listen quite as much as it possible should when drop=FALSE, meaning it is possible to end up with a numeric, rather than a 1x1 matrix
Merge pull request #20 from jefferislab/spam

Switch to using spam, not Matrix, for sparse matrices