Excel interop + newlines in cells

[joinr]

For compatibility with TSV operations, we should be cleaning up the input when we have newlines in cell entries. This creates a problem with split-lines, and we end up breaking the table parser.

Solution: strip out newlines from the cells on parse or conversion to text. use clojure.string/trim to get ride of whitespace in the cell entry. Note: this only affects xlsx file reading to tables really. Limit scope to spork.util.excel