Fix markdown autolink issues.

* Rescue C auto-linked content raised ArgumentError.

```ruby
require 'redcarpet'
markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true, :space_after_headers => true)
markdown.render 'https://github.com码'

=> "<p><a href=\"https://github.com%E7\">https://github.com\xE7</a>\xA0\x81</p>\n"
```

* Incorrectly autolink when UTF-8 character stick with the end of URL

live example:

![screenshot 2014-03-15 02 52 00](https://f.cloud.github.com/assets/1000669/2424593/bd69b3a4-aba9-11e3-8794-dd64eec011da.png)

From: http://codecampo.com/topics/822?page=2#30

```ruby
require 'redcarpet'
markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true, :space_after_headers => true)
markdown.render '（https://github.com）'

=> "<p>（<a href=\"https://github.com%EF%BC%89\">https://github.com）</a></p>\n"
```

The code is borrowed from https://github.com/ruby-china/ruby-china/blob/master/lib/markdown.rb, with minor modifications by me (English comments, minor code structure modifications.)