Move the library to its own namespace

[Andrey Semashev]

Boost.Range defines most of its symbols in the boost:: namespace. This introduces conflicts with other Boost libraries, for example:

​http://thread.gmane.org/gmane.comp.lib.boost.devel/249985/focus=249986 ​http://lists.boost.org/Archives/boost/2014/07/215581.php

Another drawback is that in user's code unqualified function calls with range arguments use boost:: namespace for symbol lookup, which may introduce unexpected results.

I propose to move all Boost.Range symbols to a dedicated namespace (e.g. boost::ranges). For backward compatibility the library can import all public interfaces except operators to the boost:: namespace.