Improving the output when exporting from the GUI to m files.

Following up on various suggestions made by @trefethen.
Fixing whitespace in Chebgui exports.
Making datestr in Chebgui exported files more appealing.
Further modifying datestr in exported Chebgui files.
Merge pull request #1287 from chebfun/fix-chebguiExporter-output

Improving the output when exporting from the GUI to m files.