exportToZip: Identifies the dependencies of an M-file and creates a ZIP file

The function "mydepfun" determines the dependencies of an
M-file, ignoring files inside the MATLAB "toolbox" directory. Skipping these files gives significant performance improvements over calling "depfun" directly, especially in MATLAB installations which include multiple toolboxes. The assumption is the same as that in "mypath.m", i.e. that most of us do our work in directories outside $matlabroot/toolbox.
The function "exportToZip" creates a ZIP file containing all the dependencies identified by "mydepfun" for one or more MATLAB functions. This simplifies the process of passing your work to another user, or uploading it to MATLAB Central, by reducing both the effort required and the likelihood of a file being missed out.

The limitations of "depfun" apply: for example, that dependencies introduced by calls such as "eval('peaks')" are not identified. An additional limitation is that if files in the MATLAB toolbox directory somehow depend on files *outside* that directory (e.g.
if you have overloaded toolbox functions), these dependencies will not be picked up as they would by "depfun" itself.

The directory hierarchy is preserved by "exportToZip", which means that all required files must be on the same drive so that a "common root directory" can be determined.