Introduced Tester.assertInstanceOf method, refs #602

| Q                    | A
| -------------         | ---
| Bug fix?          | [no]
| New feature?   | [yes]
| BC breaks?     | [no]
| Deprecations? | [no]
| Tests pass?    | [yes]
| Fixed tickets   | [#602]
| License           | MIT
| Doc PR           | [#619]

CasperJs 1.1-dev now offers [Tester Api](http://docs.casperjs.org/en/latest/modules/tester.html),
which can be useful to unit test logical business. This PR is a start to complete this API and closer to that PHPUnit offers.

- add utils.betterInstanceOf
- add many tests for native and custom objects
- add Tester Api documentation and samples