Data Type Conversion Option for RJDBC

Hi Simon,

I've added some code to a fork of RJDBC enable the conversion of data returned from queries to specific R data-types, based on the Java SQL data type constants (see [http://docs.oracle.com/javase/7/docs/api/constant-values.html#java.sql]).

Not sure if this is something you want to incorporate, but my motivation for this is that I work with a lot of time-series data and just about every one of my queries has an `as.POSIXlt` or `as.Date` immediately following it. I much prefer RJDBC over other options for two reasons - 1) It's much more portable (I work on a Windows machine where I don't have admin access :( ), and 2) I've found some of the implicit type conversion in RODBC to be too enthusiastic at times (e.g. a string column containing only numbers converted to `numeric` that I'd rather keep as `character`).

To reduce impact on existing users, I've made it the default to perform no conversions, as per the existing code.

Happy to discuss and make changes if this is of any use.