Add support for data type sql_variant

[xalopp]

Fixes #127

[meet-bhagdev]

@xalopp Thanks for the PR. We will look into this in the next few weeks.

[photodude]

@xalopp can you fix the merge conflicts here so we can try to get a PR review?

[coveralls]

Coverage remained the same at 38.071% when pulling 3182692 on xalopp:issue_127 into fe797fa on Microsoft:PHP-7.0-Linux.

[coveralls]

Coverage remained the same at 38.071% when pulling 674b237 on xalopp:issue_127 into fe797fa on Microsoft:PHP-7.0-Linux.

[coveralls]

Coverage remained the same at 38.071% when pulling 6140f44 on xalopp:issue_127 into fe797fa on Microsoft:PHP-7.0-Linux.

[xalopp]

@photodude sure. I added the test case which it's failling, but I'll still need to figure out, how to get my fix running again.

[Hadis-Knj]

@xalopp thanks for your contribution, closing this PR as it doesn't cover all cases that we need to add sql variant. #399 is the list of changes in order to support this feature. note that this feature is included in 4.2.0 release