Linux odbc library fix

On my linux system, the odbc library is `libodbc.so.2`.  I can also use simply `libodbc` from within julia, but that might be because I have the `-devel` rpm installed.  

Either way, this patch looks for the right library and sets the library name.  Since you seem to be basing `ODBC.jl` on version 1 of the library, and it still seems to work with version 2, I'm assuming later versions are backwards compatible, and so this patch checks in sequence for successive versions of the library.

This patch actually contains my previous pull, so I can fix it up if it breaks or if you decide you don't want to rename the file.  Or you can just make the changes yourself--they're self contained.

Cheers!