Lock minitest version
Implement it all in C for CRuby 2.0 and 2.1

I had to backport code from latest CRuby 2.0 and 2.1 tags. Hopefully,
all of the files are filled under their right license. Please, don't
hesitate to call me out, if they are not.
Rename gem to bindex

Exception Bindings is just too long. A bit of name creativity won't hurt
after the web-console gem. Yeah, gem was a pun :)