Don't use mem::transmute for the argument to CreateProxyHandler.
Remove htmlmainelement.rs again (re-fixes #2541).
JSON response support and some default headers
Squash a warning about deriving traits on a struct that contains a raw pointer.
Remove some code to support preffing off APIs.

We won't be using this feature in the near future, and the implementation is
not really Rustic.
Pass slices to CreateInterfaceObjects2 rather than raw pointers.
Remove the pointless terminator from ConstantSpec slices.
Store a slice in ConstantSpec, rather than a raw pointer.
auto merge of #2590 : Manishearth/servo/xhr-json, r=jdm
Remove needless '&self mut' from VirtualMethods trait.
Use Cell for mutability of HTMLIFrameElement.sandbox.
auto merge of #2600 : saneyuki/servo/mut, r=Ms2ger

Related #2514
auto merge of #2596 : Ms2ger/servo/CreateProxyHandler-transmute, r=jdm
auto merge of #2597 : Ms2ger/servo/main, r=jdm
auto merge of #2598 : Ms2ger/servo/unsafe-deriving, r=jdm
auto merge of #2599 : Ms2ger/servo/constantspec, r=jdm
Fix and disable some compile warnings.
Add an issue number to a FIXME comment.
Bonus: Make Reflector::object private.
Fix leak

Introduced by yours-truly in 7212c3573e7f2b13f907e17a5683d382b34ab932

Oopsie.