Add functionality to patch functions

[kenjis]

You can mock PHP native functions, if you set $enable_patcher = true:

$this->patchFunction('mt_rand', 100);

or

$this->patchFunction('mt_rand', function($a, $b) {
    return intval(($a + $b) / 2);
});

And if you want to reset mocked functions:

$this->resetFunctionPatches();

[kenjis]

I removed restriction on internal functions: afbcccf

[kenjis]

I changed API.
See https://github.com/kenjis/ci-phpunit-test/blob/master/docs/HowToWriteTests.md#patching-functions