Update changelog issue references with links

[mtraynham]

Simple regex find and replace:

#(\d+)

[#$1](https://github.com/dc-js/dc.js/issues/$1)

[gordonwoodhull]

I had hoped that since the changelog is hosted on github, it would use github flavored markdown, but I guess not.

I guess we really are going to need to generate it automatically.

[mtraynham]

You can use relative paths, but the issues page is not really a relative path... There might be a way to use markdown link references, but I don't know if you can concat those with strings...

http://daringfireball.net/projects/markdown/syntax#link

  [google]: http://google.com/        "Google"
  [yahoo]:  http://search.yahoo.com/  "Yahoo Search"
  [msn]:    http://search.msn.com/    "MSN Search"

[gordonwoodhull]

Yeah, apparently not:

not this ticket/845