(maint) Update README to link to extensibility document.

Updating the README to link to our extensibility document so that users
know how to extend native facter using custom and external facts.

Removing the C++ usage section from the README; I think that this makes
it seem that you extend native facter in C++, and that's not the case.
Merge pull request #235 from peterhuene/maint/readme

(maint) Update README to link to extensibility document.
(CFACT-137) Fix Unicode support

Command-line arguments aren't necessarily passed in Unicode, Boost.Log and
Boost.Filesystem use wchar_t instead of UTF-8 by default, and Ruby uses
local encoding by default. Change all of these to use UTF-8 in char.

ostringstream wasn't working correctly on Windows, so some logging didn't
appear. Also, for copying a UTF-8 string, we need the #bytesize rather than
(number of characters) #size.

Move testing to Ruby 2.1.5 on Windows to avoid a bug in Ruby's #load
method with Unicode paths, filed as ticket CFACT-170.