Allow the secret to be safely specified externally

[kohsuke]

The way the code is written today expects users to modify the file
that's under the version control system. This is inherently dangerous as
a developer needs to locally edit this file and then be very careful not
to accidentally commit the change in to the repository.

A better way to do this is to either allow these constants to be defined
through a compiler switch (for interactive dekstop development) or in a
separate file (outside the ForcePad source tree and the -I option allows
you to have this import statement resolve the header file correctly.)

This will also work better in a build server, where an in-place
modification to a file is tricky.

[jhersh]

Thanks for this! A great improvement.

Unfortunately I'm no longer at salesforce and no one there is maintaining ForcePad. I can't merge this request, but leaving it open should let others see it and use your fork.