PhpBrowser::submitForm method enh.

Added exception throw if form not found, and i made some formatting fix (changed tabs to spaces). This is PR for this issue https://github.com/Codeception/Codeception/issues/256.
Sorry for that i did not provide tests, i just got stuck with them, because when i run ```php codecept run``` then when it is turn of ```unit-tests``` some of them failing with php-error ```can not redeclare class....``` i think it is because i already has full phpunit installed and it is in my include_path. But anyway i've tested compiled phar and when form does not exist exception was correctly thrown like this: 
```
1) Couldn't test index page in Codeception\TestCase\Cest::testMySimpleApp
Codeception\Exception\TestRuntime: Form with selector: "#not-existing" was not found on given page.
```
so i think this one can be merged.