Add ChangeLog entry for custom messags handlers [skip ci]

Thanks Marc for noticing. Also tweaked the docs a bit.
Merge pull request #214 from pymssql/custom-msghandler-changelog-entry

Add ChangeLog entry for custom messags handlers [skip ci]
Made pymssql as_dict test work with SQL Server 2005. Fixes #195.
Merge pull request #215 from pymssql/issue_195

Made pymssql as_dict test work with SQL Server 2005. Fixes #195.
Make sure td_version property of _mssql layer connections always has a valid value.

It was None when client app asked for TDS version 7.2. Fixes #211.