Including tr1:: headers in ext/algorithm

[Paolo Carlini]

Dhruv Matani wrote:
> Is it fine to include the tr1:: headers in ext/algorithm???? I'll be
> needing them for tr1::unordered_map(hash table).
All in all, I still believe it's better to start a separate ext/ header. 
And now we have one more reason: we don't want to pollute headers 
containing legacy HP / SGI features with un-uglifyied names whereas we 
can do that for new extension headers.

Paolo.