Add /coverage/ to .gitignore

Prevent results of istanbul code coverage from being commited to github.
Rename server.test.js to rest.test.js
Clean up rest tests, cover prototype methods too

Refactor out helper methods
- `givenSharedStaticMethod()`
- `givenSharedPrototypeMethod()`

Add tests for calls of prototype methods to get a better test coverage before
starting refactoring of RestAdapter.prototype.createHandler.
Clean up `RestAdapter.prototype.createHandler()`

Extract smaller methods, remove code duplication, introduce
`RemoteObjects.prototype.invokeMethodInContext()`
Include full error stack in debug logs
Add rest.before and rest.after hooks.