Protect writes to lookup with hash check

This will avoid unnecessary writes if the data has not
changed.
Merge pull request #177 from atlasapi/hashcode-write-lookup

Protect writes to lookup with hash check
Remove configuration of read preferences

We now look into the spring context to obtain a
ReadPreference object.
Merge branch 'mongo-tags' into stage

Conflicts:
	src/main/java/org/atlasapi/persistence/MongoContentPersistenceModule.java
	src/main/java/org/atlasapi/persistence/lookup/mongo/MongoLookupEntryStore.java
	src/test/java/org/atlasapi/persistence/lookup/mongo/LookupEntryHasherTest.java