Skip AutoInc columns by default when inserting data.

The new methods forInsert and forceInsertAll in JdbcProfile can be
used to change that behavior. This only affects inserting values from
the client side. Inserting from another query ignores these
restrictions.

A new capability jdbc.forceInsert describes the ability to do forced
inserts. Surprisingly, SQL Server is the only database in the Slick core
lacking that feature. There used to be more of them a few years ago.

Tests in MapperTest.testMappedEntity (now relying on the soft insert
being the default) and InsertTest.testForced. Fixes issue #27.

Review by @cvogt. I'm on vacation now for one week. Time to catch up :-)