* Adding H2 Database Support
* Making HtmlCompressor a constant so it is not created new each time it is used.
Note: It is thread-safe (see: https://code.google.com/archive/p/htmlcompressor/ "HtmlCompressor and XmlCompressor classes are considered thread safe* and can be used in multi-thread environment")
* Adds the H2 Database to the used libraries in the README.md
* Changes all invalid license headers
* Refactors many methods into an enum named DBType
These methods are:
-> getName()
-> getConfigName()
-> supportsMySQLQueries()
to a check if the DBType is H2
* KeepAliveTask splitted into separate class to remove duplicate code
* Fixes compilation error
* Refactors Database Tests into one Common class with all generic Database Tests and specific Database Tests
* Fixes some JavaDocs
* Adds license headers
* Optimizing comparisons to enum comparisons