Overload static DesiredCapabilities methods with version parameter

I find that a very common pattern of code is:
```java
DesiredCapabilities caps = DesiredCapabilities.firefox();
caps.setVersion("33.1.1");
```
Overloading the static DesiredCapabilities method with a version parameter would enable this to be replaced with:
```java
DesiredCapabilities caps = DesiredCapabilities.firefox("33.1.1");
```

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