Implement wcwidth() for calculating string column widths

[Spakman]

As noted by @shirosaki (#57 (comment)), there is a Ruby port[1] of wcwidth()[2]. This should be a better way for us to calculate character widths.

1 - https://github.com/shurizzle/ruby-wcwidth/blob/master/lib/wcwidth.rb#L96-L110
2 - http://www.kernel.org/doc/man-pages/online/pages/man3/wcwidth.3.html

[phasis68]

I have implemented another ruby version of wcwidth.

https://github.com/phasis68/rb-wcwidth/blob/master/lib/wcwidth.rb

[luislavena]

Thank you @phasis68.

Under which license is that code? If is bsd compatible, then I will push it as a module of rbreadline instead of modifying string class.

[phasis68]

The original C code is under 2-clause BSD-like license.
So my code might be under the same license.
But I don't want to claim my license, use it as you want.