add go-mssqldb tests

[bhcleek]

This PR adds support for github.com/denisenkom/go-mssqldb, a pure Go driver for SQL Server.

I've updated the README with instructions for creating the database and setting the environment variables that the tests expect to be present.

The tests expect port 1433, the default port for the default SQL Server instance, to be listening on localhost; if the port is not open, then the tests will not run. The tests run against a database named gosqltest. The username and password are specified using environment variables named GOSQLTEST_MSSQL_USER and GOSQLTEST_MSSQL_PASS, respectively. GOSQLTEST_MSSQL_USER must be set, but GOSQLTEST_MSSQL_PASS does not; the tests will use a default password of gosqltest if the environment variable is not set.

I ran these tests on an OS X machine that had port 1433 forwarded to a Vagrant machine running SQL Server.