add Gist#rawify

for anonymous users, the url returns from gist api is something like
https://gist.github.com/7216441 , and it would redirect to
https://gist.github.com/anonymous/7216441, it's not easy to get
the raw file url by scripting.

this commit make the hack to handle the HTTPRedirection, and concate
the result url with /raw to generate the raw url.
add -r option, and allow use combination with -s
add spec for Gist#rawify
modify -r to -R

gist may support recursively gist all files in a directory by `-r`
add comment for rawify
[Linux] Support the xdg-open command

This can be used to open the URL if available
Merge pull request #161 from EvanPurkhiser/feature/support-xdg-open

[Linux] Support the xdg-open command
Rebuild [Fixes #159]
Trust to gem loading [Fixes #156]
Silence interrupts more globally
Use https rubygems
Fix warnings
Handle login failure nicely [Fixes #154]
Merge branch 'add-raw-option'
Added Homebrew installation instructions
Merge pull request #163 from KB1JWQ/addhomebrew

Added Homebrew installation instructions
Updating installation info in manpage
Merge pull request #164 from KB1JWQ/addhomebrew

Updating installation info in manpage