Adding minimal support for CommonJS and Spidermonkey

I needed to perform some PGP digital signature validation inside CouchDB (spidermonkey based JS engine).

This pull requests adds minimal support for CommonJS such that one may use:

    var openpgp = require('openpgp').openpgp;

Additionally I define some minimal dependencies that don't exist in SpiderMonkey (navigator, localStorage, some jQuery, and showMessage stub) to allow loading of the library without errors.

I've made the SpiderMonkey additions a conditional with the minify step and added a separate build step.

I've only tested the public key import, and signature validation.

You may license my contribution to coincide with the project as LGPL, but should include the following statement:

    Copyright 2012 SRI International
    
    This project has been funded at least or in part with Federal funds from the U.S. Department of Education under Contract Number ED-04-CO-0040/0010. The content of this publication does not necessarily reflect the views or policies of the U.S. Department of Education nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.