Support rvalues in adaptors::indexed

Using a normal ranged for with an rvalue is supported in C++, so adaptors::indexed should support it too. 

Currently it is too easy to get a segfault:

```
std::vector<int> getVec();
for (const auto& i : boost::adaptors::index(getVec())) { std::cout << i; }
```

This should have a move constructor and be able to store the temporary object.