Increased story description size

Stories can now have much longer descriptions.
The database is updated with the new column length at startup.
Created a simple framework for handling schema updates

Classes in the dbupdate package that extend DbUpdater and have the
annotation DbUpdate will get registered as schema updates and will
be run at startup. The first update increases the length of story
descriptions.
Added some comments about the schema update framework

Also added an exception if the database type was unknown.
Longer description for epics and themes

Increased the description size for themes and epics as well.
The size has been set to a maximum of 100000 characters, which
should be more than enough.
Merge branch 'master' into longer-description

Conflicts:
	pom.xml
Merge pull request #78 from fredrikpersson/longer-description

Increased story description size