introduce new assertInstanceOf() method

When you need to unit test your front logical business, instanceof become useful to be sure to have the good object.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Tests pass?   | yes
| Doc & Tests         | of course !

Tested on GNU/Linux Mint 14 with PhantomJS 1.9.1 & casperJs 1.1-dev