BUG: Fix smaple_names being casted to int/float

read_csv was casting the `'sample_name'` column to a float if the values were
of the form `0000012.122204`, this has been fixed and we have also added a new
keyword argument to skip lines that are "empty" or that lack a sample_name i.
e. if a line starts with a `\t` character it will be ignored. Empty columns
are also now removed.
Hmmm, I was under the impression that # was only a comment character for QIIME mapping files.