Fix compilation error with as_deque in C++11.

In C++11, `as_deque` did not compile for even simple use cases. I added a unit test and a fix (which is not 100% clean, see inline comments). With the patch applied, all the unit tests are passing on develop with Apple's Clang and -std=c++1y.
This constructor hides the `Sequence const&` constructor below in the file. When trying to construct a deque from a sequence, this will fail because we'll take the sequence itself as being an element of the deque. So basically, I added a `enable_if` which disables this constructor if the `Head_` type can't be converted into the first type in the deque. A more complete solution would be to check whether `Head_` and all of `Tail_...` can be converted to the corresponding deque type.