Added support for NOT using reverse complements when computing the similarity matrix

Pull request here: #38

To avoid using reverse complements (e.g. if working with splicing motifs), set the argument revcomp=False when calling a TfModiscoWorkflow instance on your data. If reloading a saved TfModisco results object, then you also have to set revcomp=False when calling prep_track_set. Otherwise, the revcomp argument is by default True (for backwards compatibility). Permalink to a notebook demonstrating the functionality is here: https://github.com/kundajelab/tfmodisco/blob/d88a1dba7f59f6dc8f62aa267ac42eb5e53037d4/test/nb_test/NoRevcomp.ipynb