Concurrency check added in Document Table

When you try to change document that just has been changed by someone
else, it throws a
[StaleObjectStateException](http://docs.jboss.org/hibernate/orm/4.3/javadocs/org/hibernate/StaleObjectStateException.html).
See the
[test](https://github.com/PreXident/TextAn/blob/master/Data/src/test/java/cz/cuni/mff/ufal/textan/data/test/ConcurrencyTest.java).
I thing that's enough for adding/updating documents co it fixes #19
You need to recreate database as described in #28