Make saving the library easier.

[3rd-Eden]

Manually saving the library can a bit of a pain. We should make it easier to developers to save the library to disk.

1. We can add a /primus.js route and send the compiled library. This would be great for testing and makes it significantly easier to get started with primus.
2. Another option would be adding Primus#save(path) method where we will just save the compiled library to disk.

[nicokaiser]

Instead of /primus.js maybe Primus.pathname + '.js' or Primus.pathname + '/client.js'?

This way, everything related is contained in the path in Primus.pathname.

[3rd-Eden]

Decided to for /<pathname>/primus.js so it's still namespaced as you suggest but has name that is easy to remember.