Scripting support - modify documents during reindex, query into, import

Added scripting support.  Can add script elements to modify data on reindex, query into, import, ...  Based on similar model in update by query.

This is applied to the other pull request for 1.0.0.beta1 compatibility, which was based on 0.90.3 branch.

Tests were added for scripting, documentation in the README is also updated.