Add apply method to Endpoint companion

This allows for endpoint to be created with slightly prettier syntax.

Addresses #88
That is what I did initially, and I get this compiler warning:
```
[warn] /home/ben/git/finch/finch-core/src/main/scala/io/finch/Endpoint.scala:144: method route does nothing other than call itself recursively
[warn]     new Endpoint[Req, Rep] { def route = route }
[warn]                                          ^
[warn] one warning found
```
Happy to chose any other identifier. I agree that underscores in public apis is not ideal.