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 
Agree yes. @ruflin we can handle this in a separate issue if you would like to "cleanup" the tests as @dbu mentioned. Not highly needed, just a bit more work.