Development version 2017-02-04

• Added print.alignment and print.graphic arguments to SpectrumSimilarity, providing options to generate these outputs. Added x.threshold argument to SepectrumSimilarity to exclude a lower portion of the spectrum from the similarity score calculation.
• Added output.list argument to SpectrumSimilarity, providing the option to return the similarity score, alignment dataframe, and plot in a list (thanks to M. Witting). In this case the plot is a grid graphics object.
• Added remove.placeholders argument to ReadMSPFile and ReadMSPDirectory to optionally remove zero intensity peaks from the resulting data frame.
• Included iodine in ListFormula and MolecularWeight (thanks to G. Getzinger).