DATASOLR-88 - Support for index time boost on managed entites.

Boosting Documents/Enities at index time is now possible using 'Boost' annotation processed by 'MappingSolrConverter'. Values can be applied on both 'Document' and 'Field' level.

CLA: 61520130807033621 (Francisco Spaeth)
Original Pull Request: #34
DATASOLR-88 - Polishing of support for index time boosting.

Additional tests for 'PersistentProperty' and 'PersistentEntity' added in order to assert 'Boost' value is processed correctly before applying it to SolrDocuments.

Removed 'ElementType.TYPE' from 'Boost' in favor of 'SolrDocument(boost)'.
Removed 'ElementType.FIELD' from 'Boost' in favor of 'Indexed(boost)'.
Added 'boost' to 'Indexed' for more convenience.

Fixed minor bug on processing 'PersistentProperty.isReadonly'.