Omit public keyword in tests on Groovy.

From http://groovy.codehaus.org/Groovy+style+and+language+feature+guidelines+for+Java+developers

"By default, Groovy considers classes and methods 'public'. So you don't have to use the 'public'
modifier everywhere something is public. Only if it's not public, you should put a visibility
modifier."

No functional changes.
DatabaseImagePersistenceStrategy: introduced strategy for persisting/getting images from database.

Addressed to #132

No functional changes.
DatabaseImagePersistenceStrategy.save(): moved method from ImageServiceImpl.save().

Addressed to #132

No functional changes.
Split liquibase changelogs according to best practices.

Inspired by http://habrahabr.ru/post/179425/ (RU)

No functional changes.

Not tested!

[ci skip]