unicode support in boost::filesystem

[sdanil]

- It is looks like most of modern file systems works in
UNICODE. 

- There is a trivial transformation method from UNICODE
to the low part of ASCII. And if there is no UNICODE
support in some filesystem, then it can be emulated
using this transformation.

- The inaccessibility of UNICODE support breaks all the
advantages of boost::filesystem, because if you live
outside USA, UK and other English-speaking countries,
then you must be ready to work with UNICODE .paths