Use channel as an input_iterator

[schaumb]

Currently, the channel iterators can be used only as output iterators.

Integrates well with STL algorithms.

If I want to pipe two channels, there is no support for the STL algorithms:

msd::channel<int> one;
msd::channel<int> two;
std::copy(one.begin(), one.end(), two.end());

[andreiavrammsd]

I'll leave this open for a while to see if I get to it.