Toggle between Class.php and ClassTest.php with one shortcut

Hi,

I added two features:

* Use one shortcut to switch from Class.php to ClassTest.php and vice versa.
* Do a matching between the class and the file.

For example my class `micClass` is matching the file `miClass.php`. 
Therefore, I had two (facultative) entries in `PHPUnit.sublime-settings`
````
{
    "prefix_file": "mi",
    "prefix_class": "mic"
}
````

Hope, this could help.