Adjust driver to work with DataNucleus

I needed to make some changes to the SQLDroid in order to use it with DataNucleus for Android version of my application. After these changes application passes test, in which DataNucleus automatically creates a few tables and after that it makes a couple of inserts and selects. My work was based on the efforts of my friend, who created this pull request - https://github.com/SQLDroid/SQLDroid/pull/39. So changes in SQLDroidDatabaseMetadata are based on Xerial JDBC Driver. I mostly modified methods that had been not implemented yet. SQLDroidClob is based on ClobImpl of DataNucleus, details in its javadoc.