Allow MySQL database to be configured in settings.xml or pom.xml -- close #13

Add maven parameters for db-{user,password,database,host}

load info from cbi.versionchecker.db.* properties by default.

create mysqldbi with user-specified parameters

change MvnP2Util(DBI) from protected to public, to allow plugin to insert its dbi.

generate url in abstractversionmojo based on host and database configured by the user.
document how to configure mysql