Fix accidental type conversion in sluggable behavior.

This generated code:

    $slugNum = substr($object->getAccountId(), strlen($slug) + 1);
    if (0 == $slugNum[0]) {
        $slugNum[0] = 1;
    }

    return $slug2 . ($slugNum + 1);

will turn `$slugNum` into an array if substr returns false. By checking
with `===` against `'0'`, this is avoided.
Fixed diff task combined with skipSql

Do not compare modified or renamed tables with skipSql option.
Added unit test for modified table case

Added unit test for modified table case and removed check in renamed
table case due to the impossibility because of previous two checks
Merge pull request #762 from javer/patch-diff-skipsql

Fixed diff task combined with skipSql
fix DebugPDOStatement for usage with execute
Merge pull request #770 from jmglsn/master

DebugPDOStatement on execute()
reworked approach for better handling of executed queries in DebugPDO
Merge pull request #775 from thedave80/DebugPDO-fix

DebugPDO executedQueryString - quoting
Merge pull request #765 from rvitaliy/connection-fix

fixed a getter function without '$con'
Merge pull request #766 from rvitaliy/doAdd-fix

Added type-hinting on doAdd crossFK generated functions
Merge pull request #707 from gunnarlium/bugfix-sluggable-fails-when-string-is-empty

Fix accidental type conversion in sluggable behavior.
Fix CS
Prepare 1.7.0 release
Bump version to 1.7.1-dev
Allow gh-pages to be tested
Update links to the 1.x documentation
Merge pull request #781 from j0k3r/patch-1

Update links to the 1.x documentation
fixed type-hinting in addDefaultMutator and added tests for it
wip