Migrate to Java 8

[svenefftinge]

In order to make contributions and usage as simple as possible for Java devs, we should migrate our code base to Java 8. (see comments like #25 (comment))

For the protocol types we should still use a code generator written in Xtend (just because using Java here would be too painful) but the actual implementation should be pure Java.

Also once there is a formal definition of the protocol (see microsoft/language-server-protocol#40) we should use that as the source for our code generator.

[svenefftinge]

While reworking the code for lsp4j, with the main two goals

1. better separation of jsonrpc and lsp
2. easier custom extendability of LSP
   I write all the new code in J8. There are still some left over, which we will translate in the coming days.

[spoenemann]

Finished the conversion for the service / jsonrpc code. The protocol data structures are still generated with Xtend. How this is going to be done in the future is discussed in #49.