Fix bug and make a number of other changes

In detail:

  * Fix bug: handle large integers correctly.

  * Save number as integer whenever possible.

  * Change NULL to the empty string if:

    * the file is a CSV file or;

    * the "-t" flag is given.

  * Document "-t" flag.

  * Update PHPExcel to 1.8.0 and remove workarounds
  necessitated by a bug in PHPExcel 1.7.

  * Add file stating licence.