reorganization to make it possible to run in the browser

[miguelgrinberg]

This change removes or makes optional the use of Node-specific elements, so that the library can also be used in a browser context.

More specifically:

• Templates are pre-compiled to a .js file at build time, so that they do not need to be loaded from files at runtime.
• Exception to the above, during tests the templates are still read from disk, so that Jest's watch option can catch updates.
• The SubprocessExporter emits an error if it is used in the browser, because it needs exec(), which is only available in Node.
• The list of HTTP methods is hardcoded, instead of importing it from the http module.

[JoshMock]

A few nits but LGTM!

[JoshMock]

nit: you can use path.join to avoid having to do this detection yourself.

[miguelgrinberg]

Yes, but I would need a polyfill for path for the browser. So far I managed to get everything working with only a polyfill for url, but I should probably try to eliminate that one as well.