DATAMONGO-607 - Introduced FieldNamingStrategy SPI interface.

MongoMappingContext can now get a FieldNamingStrategy configured to allow the customization of field names used to persist property values to in case *no manual mapping is defined* (e.g. through @Field). The default strategy will simply use the property name as it did before.

We now also expose a abbreviate-field-names attribute on the <mongo:mapping-context /> XML namespace element to transparently register a CamelCaseAbbreviatingFieldNamingStrategy which abbreviates the property's name to the first letters of its camel-case structure. A property fooBar would then be persisted to a field named fb. If you're not using the XML namespace simply configure the strategy on your MongoMappingContext instance.

To avoid field name mapping ambiguities being introduced through a custom FieldNamingStrategy (imagine the camel-case strategy just mentioned and two properties lastname and level which would both map to l) the PersistentEntity implementation verifies the mapping metadata and throws an exception in case an ambiguity is found.