4.4: support XHP codegen, require HHVM 4.80, support HSL 4.53

Add support for codegen xhp classes (#138)

* Add support for codegen xhp classes
Adds xhp attribute codegen to traits and classes.
No error is thrown when attributes are added to non-xhp classes.
This syntax is valid, but not useful in the current class.
Inheritance maybe?

* Fix broken copy paste

* Compatibility with older hsl versions

* Whitespace lint

* Update xhp attribute doc

I copy pasted property and did not touch up the doc well enough.