Clean up the main module and add a search_class attribute to Alchemy

[sascha-karnatz]

1. remove Search module
   • the main module was refrencing to a few methods in the Search module which was some kind of duplication
   • it was hard to understand, why the Search module existed in the first place, that the main module contained a few function itself
2. add a search_class configuration to Alchemy namespace
   • Allow the configuration of an own search class and overwrite the default behavior without monkey patching