Update generate.php with compression

Generatedata sometimes provides files that are several MB in size. In order to save on bandwidth costs, data files over 500 KB will be compressed to zip. 100000 rows of data consisting of about 5 columns of data is about 6 MB uncompressed, and a little over 2 MB when not. That's over 60% bandwidth saved.

It would be great if we could add the compress option to the UI so that users would have a choice.
Update generate.tab1.tpl

Added the checkbox for the compression option
Update generate.php

Script picks up the compression option for the POST data. This seems to be working with most export types except for Excel (generates the excel file irrespective) and Language types (blank zip archive).
Update generate.php

Corrected mistake.
Update generate.php

All file types can be compressed now except for excel.
Update DataGenerator.class.php

Added the private data compression flag to this class. Coded constructor to initialize this flag based on POST data. Also included a public getter method for the compression flag.
Update Excel.class.php

Attempted to compress Excel Data. No luck so far!
Merge pull request #169 from unullmass/master

Added Compression Feature