Mysql dialect

Implements the Dialect interface for a MySQL database, using the following sql driver:

github.com/go-sql-driver/mysql

Error translation

Because the driver only passes the numeric error codes, we had to redefine all the error constants in errors.go. This is done automatically from the mysql headers by running the following command in the current directory:

go generate

The script "tools/generrors.go" is doing the actual job of writing the source file. It requires the headers to be installed, and was last executed with libmysqlclient-dev 5.7.16-0ubuntu0.16.04.1 on a ubuntu 16.04.