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`
   - Solr 4.0 will *corrupt* existing documents when trying to do atomic updates (see [SOLR-4127](https://issues.apache.org/jira/browse/SOLR-4127))
   - Solr 4.1 and newer will deny the update attempt and reply with a `400 Bad Request`, mentioning that `<updateLog />`
 needs to be configured.
- A `_version_` field must be defined in the Solr schema.
- All fields in the Solr schema must be defined with `stored=True`

**Caveats:**
- If there's a field in the Solr schema that's *not* `stored=True`, it will get **dropped** from documents in Solr on the next update to that document. Indexing won't fail, but that field simply won't have any content any more.

**Tested (in principle) with:**
- Solr 4.1
- Solr 4.2
- Solr 4.3
- Solr 4.4
 
Also see http://wiki.apache.org/solr/Atomic_Updates