pmd.xml: updated URLs to PMD's tickets.

No functional changes.
Added support of Liquibase for database migrations.

Most usefull commands:
- mvn -P dev liquibase:status
- mvn -P dev liquibase:update (or liquibase:updateSQL)
- mvn -P dev liquibase:rollback (or liquibase:rollbackSQL) -Dliquibase.rollbackCount=N
- mvn -P dev liquibase:updateTestingRollback

@see http://www.liquibase.org/manual/maven
@see http://www.liquibase.org/manual/changeset
@see http://www.liquibase.org/manual/refactoring_commands

Fix #139
Removed default values of <link> type attribute.

Inspired by:
http://net.tutsplus.com/tutorials/html-css-techniques/25-html5-features-tips-and-techniques-you-must-know/

HTML5 migration.