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.
Merge pull request #725 from selendroid/refactor/package-structure

Introducing package per component.