Implemented Modal helper
Minor HtmlElement fixes

* Documentation on prependContent
* Removed the executable bit
Fix whitespace errors

* DOS line endings violate FIG standards
* Replaced accidental tabs with spaces
Added string cast to __toString method

This was suggested by @ronaldbaltus. The reasoning was that
the render() method does not neccesarily return string, but
could instead rely on __toString to do the final conversion.
Allow passing a ViewModel

Following an awesome suggestion by @RWOverdijk himself.
Moved maybeRender() to Modal
Merge pull request #61 from paul-vg/feature/modal-helper

Modal helper