[Proposal] Fix for HtmlBuilder and FormBuilder for correct xHTML

I need this for my project where I am using xHTML MP 1.2. Since I am quite new to Laravel, I am not sure whether it is a good idea to implement that stuff like I  did.

In addition, I created a doctype function for just shorter syntax.

In my view, the config `app/config/html.php` should look like this:
```php
return array(

	/*
	|--------------------------------------------------------------------------
	| Markup
	|--------------------------------------------------------------------------
	|
	| Here you may specify the markup for HTML Builder and Form builder for
	| correct tags rendering.
	|
	| Supported: 'html', 'xhtml'
	|
	*/

	'markup' => 'html',


	/*
	|--------------------------------------------------------------------------
	| Document Type Declaration
	|--------------------------------------------------------------------------
	|
	| Here you can specify the default document type, which will be used
	| with HTML::doctype
	|
	*/

	'doctype' => 'html5',


	/*
	|--------------------------------------------------------------------------
	| List Of Doctypes
	|--------------------------------------------------------------------------
	|
	| Taken from:
	|  - http://www.w3schools.com/tags/tag_doctype.asp
	|  - http://en.wikipedia.org/wiki/Document_type_declaration
	|
	*/

	'doctypes' => array(

		'html5'              => '<!DOCTYPE html>',

		'html-strict'        => '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">',
		'html-transitional'  => '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">',
		'html-frameset'      => '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">',

		'xhtml'              => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">',
		'xhtml-strict'       => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">',
		'xhtml-transitional' => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">',
		'xhtml-frameset'     => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">',

		'xhtml-basic'        => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.1//EN" "http://www.w3.org/TR/xhtml-basic/xhtml-basic11.dtd">',
		'xhtml-mp10'         => '<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.0//EN" "http://www.wapforum.org/DTD/xhtml-mobile10.dtd">',
		'xhtml-mp11'         => '<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.1//EN" "http://www.openmobilealliance.org/tech/DTD/xhtml-mobile11.dtd">',
		'xhtml-mp12'         => '<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.2//EN" "http://www.openmobilealliance.org/tech/DTD/xhtml-mobile12.dtd">'

	)

);
```

Tests included.