Use kr's PTY for improved OS X behavior

This changes the PTY source over to the upstream by kr, as he's added
working Darwin support.
Merge pull request #261 from antifuchs/use-kr-pty

Use kr's PTY for improved OS X client behavior
Better regexp for rspec files
Merge pull request #263 from PugglePay/better-rspec-regexp

Better regexp for rspec files (Issue: #262)
Fixed relative links in README
Fixed more links
Fix links in docs/ruby/modifying.md
Merge pull request #264 from albertyw/master

Relative Link Fixes in Documentation