Add support for atomic updates.

This change implements the capability to perform atomic updates of documents in Solr.

This means whenever possible, only the necessary / specified attributes get updated in Solr, and more importantly, re-indexed by Plone's indexers.

**Prerequisites:**
- An `<updateLog />` must be enabled in `solrconfig.xml`
- A `_version_` field must be defined in the Solr schema.

See http://wiki.apache.org/solr/Atomic_Updates