Migrate file structure to PSR-4

Initial proposal:
- move `lib/Elastica` to `lib`
- move `test/lib/Elastica/Test` to `test/lib`

Let me now if there are other opinions in directory restructuring.

Resolves #715 
most libraries i see use just `tests/` not followed by "lib" for this. 

btw, it seems to be a convention that tests are in the namespace "Tests". if there is "Test", i would expect it to provide testing helpers that i could use for my tests as a library user (like a mock elasticsearch client or something). but changing that has a lot more potential for BC breaks.