URL: Support protcol-relative URLs

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Type the URL in this format "//url".
2. Observe the result is not parsed as a valid URL.


What is the expected output? What do you see instead?
Support protocol-relative URL. The result is not parsed as URL.

What version of the product are you using? On what operating system?
v 1.2
Tested .md file in Visual Studio 2013 using Web Essentials v1.5.
Windows 8.1

Please provide any additional information below.
According to the RFC 3986; "//" should get resolved as the default scheme, 
section 5.4:


> "//g" = "http://g"


PS: please consider signing your assembly (MarkdownSharp.dll) with version 
numbers. The Version in VS's property window is empty (when selected under 
References).

Thanks.

Original issue reported on code.google.com by adeel...@gmail.com on 30 Dec 2013 at 3:09