Add Compatibility to DDBC

Hello, It would be great to have an interface class similar to JDBC. That's why I create DDBC, which aims to be the equivalent to JDBC for Dart. DDBC is composed of three abstract classes:
 1. Connection
 2. ConnectionPool

And it is also composed from next classes:
 1. Result
 2. Row

Those four classes should be compatible with others DDBC implementations, so that we could use other libraries without changing the final user code.

Furthermore, I also create exceptions.dart file that should mantain the compatibility layer for sql exceptions.

And finally, I create sql_formatter.dart which is in charge of formatting and escaping sql before sending it to the database.

I also going to try to make compatible SqlJocky library.