API Remove search and Solr classes, moved to cwp-search module

[robbieaverill]

This moves the search/Solr related code from this module into the cwp-search module, and updates the documentation.

Related pull requests:

• cwp: https://github.com/silverstripe/cwp/pull/30
• cwp-search: https://github.com/silverstripe/cwp-search/pull/1

Resolves #14