Fix date interval database value truncation (string overflow)

Add mysql versions 5.6 and 5.7 to travis-ci

Fixes #2755
Fixes #2757

Fix documentation format regarding retryable exceptions

Add documentation regarding transactional exceptions

Move coverage session to sqlite.travis.xml file

Generate code coverage via phpunit

Add scrutinizer to the CI

Close #2730

Require PHP 7.1

Connection: Support IS NULL in update/delete criteria

If you pass a null to the where array of update or delete it's
pretty obvious what you want.

Most (all?) databases won't match anything if you actually try
to compare with NULL making this a silent failure without it.

fixed array declarations

Correct param comment

The param comment is currently set to string, which is wrong.
The correct hint would be integer/int

Fixed all array declarations inside Driver dir

Correct classes referenced in the Driver implementation details in the documentation

One class missed and one non-existing in example at "Supporting Other Databases" page.

Normalize method signatures for `fetch()` and `fetchAll()`

Wrong class used in documentation - referenced `Driver` instead of `Driver\Connection`

At lest it looks like one to me.