[Feature] Integration with beberlei/assert library

[lisachenko]

Integration with @beberlei's assert library can give a nice tool for writing DbC conditions and assertions, see https://github.com/beberlei/assert#list-of-assertions. For example,

    /**
     * Deposits fixed amount of money to the account
     *
     * @param float $amount
     *
     * @Contract\Ensure("Assert\Assertion::integer($this->balance)")
     */
    public function deposit($amount)
    {
        $this->balance += $amount;
    }

[pdaw]

@lisachenko

Good idea, I will create pull request with integration soon.

[lisachenko]

@pdaw Thanks for contribution! Released tag 0.3.0 with minor changes in your code.