added H2 database support

On my installation (openjdk7 on freebsd 9.2), LWC caused the JVM to crash with a SIGSEGV fault. Apparently because of the native libraries used by sqlite. 
Not wanting the hassle with an external mysql database, i added support for the pure-java database H2.

It appears to be a drop-in replace for the existing databases, with comparable performance to sqlite.