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Natural::shrink respect lower limit
I broke shrinking and there was no test to catch it, so I added it
Fixed bug in Generator\Natural shrinking: values were going under the lower limit
Merge pull request #20 from giorgiosironi/natural-lower-limit

Natural::shrink respect lower limit
Unifying tests for Generator\Natural after network partition
Generator\Integer
Added failing test for uniformity of distribution
Robust check for $lowerLimit and $upperLimit

Since we can't use type hinting for scalar values we have to manually
check the boundaries of the domain in the constructor.
Generating Integer values from the interval [0, size]
Avoid overflow when generating negative integers with PHP_MIN_INT as
boundary.

Signed integers goes from −9,223,372,036,854,775,808 to
9,223,372,036,854,775,807 on a 64 bits system.

The generator algorithm was doing this:

Context:
$upperLimit = 0;
$lowerLimit = −9,223,372,036,854,775,808;

rand(0, ($this->upperLimit - $this->lowerLimit));

which was causing a 0 + 9,223,372,036,854,775,808 which is a clear
overflow.
Ensured lowerLimit < upperLimit and that it is not possible to go out of
the domain during the shrink.
Checking that Generator\Integer:shrink/1 argument belongs to the domain
before shrink it
Natural lowerLimit must be greater than 0

Added use for `InvalidArgumentException`
Strictly requiring PHPUnit 5.4
Natural lowerLimit must be lower than the upperLimit
Better error messages for StringTest, failing on some versions of PHP randomically
Natural limits must be Integers
Natural cannot shrink values that does not belongs to the domain