Added a few options to delete, update Solr docs and other options

Hi,

I made a lot of changes to use features included in Solr 4.x and up. These changes are:

1. Added  updateDoc method
2. Added a deleteDoc method (a single doc or an array of ids to delete)
3. Added a deleteDocByQuery (remove docs matching the query)
4. Added a method to delete a doc field

In the addDoc readme you are using addField but I think it should be setField?

I tested my code and updated the README. Let me know if you need some other clarification.

Thanks,

Harry