Introducing package per component.

This commit is having no functional change, it is introducing a defined package per module.
client:          --> io.selendroid.client
grid:             --> io.selendroid.grid
server:         --> io.selendroid.server
standalone: --> io.selendroid.standalone

Customers using the new version have to refactor their code, but it is necessary and leads to cleaner code.

So therefore it is the right thing to do.

@mkonicek please review, merge and delete branch.

fyi: @lukeis @freynaud @mach6 @operatiay @jlipps