Indexer.__init__() Sometimes Calls Series.dropna()

When an indexer wants a list of Series, but is given a DataFrame, we call
dropna() to get rid of the NaN values that were introduced while making
the DataFrame. Otherwise, they'd cause problems, and they aren't even
relevant.
HorizontalIntervalIndexer Returns a DataFrame

Working toward issue #281
update default 'return' line in TemplateIndexer.run()
NoteBeatStrengthIndexer Returns a DataFrame

Working toward issue #281