Fixed random TestableDBActionTest failures by forcing org.h2.Driver initialization.

Before this fix, the TestableDBActionSpec depended on a previous H2 driver initialization. The result of the TestableDBActionSpec depended on race condition with DBSpec.
When using a JDBC driver not known by Play-Slick, user can now specify the Slick driver explicitly.

For example, when using MariaDB driver, one may add:

db.default.slickdriver="scala.slick.driver.MySQLDriver"
If an arbitrary driver is specified, it is preferred to the default one.

Many people probably will not care about that. There is probably no rational reason to use PostgreSQL driver for MySQL :) However, there are several reasons to do this change:
1. It is more consistent and predictable.
2. One may want to use an enhanced or a patched driver.
Test for nested objects. I was not sure if scalas generates the MODULE$ field in this case.
The tests should be more readable now.

There were many names for databases. These were chosen too quickly. I've renamed the databases to be more meaningful.
Merge pull request #110 from v6ak/master

 When using a JDBC driver not known by Play-Slick, user can now specify the Slick driver explicitly.