Add some hooks to JdbcMappingCompilerComponent.

This makes it easier to change the mappings in driver implementations.
OracleDriver needs to hook into the mapping compiler now to work around
Oracle't strange NULL string handling because the Option handling logic
moved from JdbcType into ResultConverter.
Merge pull request #770 from slick/tmp/mapping-compiler-hooks

Add some hooks to JdbcMappingCompilerComponent.