Simplified will return value

As proposed in #144 by @ciaranmcnulty and @whatthejeff here's another PR for a simplified use of `will($this->returnValue($value))`:

```
$stub->method('doSomething')->willReturn(...);
$stub->method('doSomething')->willReturnMap(...);
$stub->method('doSomething')->willReturnArgument(...);
$stub->method('doSomething')->willReturnCallback(...);
$stub->method('doSomething')->willReturnSelf(...);
$stub->method('doSomething')->willReturnOnConsecutiveCalls(...);
$stub->method('doSomething')->willThrowException(...);
```