PartialFor implementation

[zabulus]

I've made all refactoring I wanted.
Breaking change I've mentioned it is extracting ISection interface from Section class.
I've made it because needed to reimplement Section extension methods in PartialProxySection. Now it need only to implement ISection interface, all extension methods we'll be added.

Fixed ViewData to be more strongly typed, though some reflection is used for this.

PS Sorry for the mess of PRs.

[zabulus]

I've found several bugs in my implementation.

1. Looks like binding from partial views doesn't work at all if PartialFor used. For now I'm sure that calling EditorFor for is not sufficient to work.
2. BeginSection in child partial works wrong if use Form.PartialFor, Looks like it writes to another TextWriter.

[zabulus]

Fixing the HtmlHelper in ProxyForm looks like fixes issue number 2.
Though can't write test for all this issues.

[robdmoore]

#104