prefix internal module names with proj4/

Hi, things are looking pretty good, keep up the good work!  I do have one suggestion (implemented here) for the AMD module structure.  With the way things are set up now, the `amd/proj4.js` build will declare a number of its internal modules (`extend`, `common`, `core` etc.) as top-level modules.  This effectively precludes these names from being used for other modules in an AMD project.  I suggest prefixing the modules with `proj4/` to avoid this.  This pull request accomplishes this.  In order to avoid a bunch of paths configuration (considered an anti-pattern in AMD), I simply renamed the `src` folder to `proj4` and moved the primary `proj4.js` module to the top level of the repo.  Let me know what you think.

The internal implementation is unchanged and I verified that this passes the tests.