Optional enums

[JoakimLofgren]

I made a suggested solution to my issue #15.

I updated the Enum constraint to take the required attribute into consideration
when checking an enumeration property and whether a value exists or not for it.

I also refactored it a bit to minimize the code.
The use of in_array should pretty much mimic the "foreach-if-equal-then-found".

Test cases are added to reflect the changes.

I hope everything is satisfactory. :)

Only one unrelated test case is failing, and it must be because
I am using Windows 7 with PHP 5.4.5 at home.

There was 1 error:

1) JsonSchema\Tests\Uri\UriRetrieverTest::testResolveRelativeUri with data set #0 ('{
    "type":"object",
    "title":"child",
    "extends":"http://some.host.at/somewhere/parent",
    "properties":
    {
        "childProp":
        {
            "type":"string"
        }
    }
}', '{
    "type":"object",
    "title":"parent",
    "properties":
    {
        "parentProp":
        {
            "type":"boolean"
        }
    }
}')
JsonSchema\Exception\UriResolverException: Unable to resolve URI 'grandparent' from base '/somewhere/parent'
...
FAILURES!
Tests: 147, Assertions: 153, Errors: 1.

[justinrainbow]

Looks great - all tests passing, too