ability to override default module path

Hello!

I wanted to be able to specify the path for loading modules in the various connection managers, in order to be able to use a locally installed database driver.  I added a simple configuration option `dialectModulePath` that takes a string representing the path to be required, and modified the various 

I believe I followed the style guidelines, but please let me know if I should make any additional changes.  Thanks a lot for this great project!