mssql: add support for a user supplied msghandler

As things stand, it is impossible to gain access to messages emitted
from the server from a stored proc. Add support in _mssql for providing
your own msghandler (which gets executed from pymssql's msghandler).
Merge branch 'master' into msghandler_support
Add tests and docs.
Try to make tests work on Python 3.x.
Merge pull request #139 from pymssql/msghandler_support

mssql: add support for a user supplied msghandler
Docs: Specify SQL Server 2005 is the oldest version supported.

Fixes #198.
Add survey results links to docs

Adds links to Excel and PDF versions of survey results
Merge pull request #213 from pymssql/add_survey_results_links_to_docs

Add survey results links to docs
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 tds_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.
Merge pull request #217 from pymssql/issue_211

Make sure tds_version property of _mssql layer connections always has a ...
docs: Add a couple of new features to the intro document.
docs: Fixed title-related reST errors after b26c3df844.
Merge pull request #221 from pymssql/add-features-to-intro-doc

docs: Add a couple of new features to the intro document.
Merge pull request #222 from pymssql/fix-docs-index-html-headings

docs: Fixed title-related reST errors after b26c3df844.
Add documentation build step to Travis CI script.

Fixes #163.
Merge pull request #223 from pymssql/163-add-docs-building-to-travis

Add documentation build step to Travis CI script.