Added support for "Put Mapping" API

See: http://www.elasticsearch.org/guide/reference/api/admin-indices-put-mapping.html

Related: #645, #490, #194

Closes #520
Refactored the "Put Mapping" API support

* Cleaned up and simplified code
* Made the return values more compatible with the rest of the API (return `false` for failures, users should check `index.response`)
* Removed `put_mapping!`
* Amended, completed and polished tests
* Updated documentation

Related: #194, #490, #520, #645
Added support for "Delete mapping" API

Fixes: #520
Refactored the "Delete mapping" API support

Related: #520