Add initial-pool-size config to c3p0 configs

Being able to configure the initial pool size in c3p0 is pretty convenient and I'd love to be able to expose that through Korma. For reference: http://www.mchange.com/projects/c3p0/#initialPoolSize

Please note that I have no idea how to run the UTs on this project, so let me know if there's a straightforward way of trying that out. I ran the full UT suite of our API using this custom version of Korma and had no trouble and was able to verify intended behavior.