Create a listing of all packages

This creates a page as part of the julia documentation suite with  the details of currently available packages as specified in the METADATA. As requested in #1896

There are a couple of issues for comments

1. This needs a Github OAuth token, since un-authenticated requests are limited to 60 per hour, which is not enough given the number of packages we have. Therefore, you need to specify the token as an environment variable GH_AUTH for the script to work. 

  Once the auth token is specified, run `make listpkg` to create the latest listing. Note that this will update your metadata repository, and install the JSON package if you dont have it yet. 


2.  I have checked in the resultant packagelist.rst. This is up for debate, but I thought that the requirements above should not be made mandatory for everyone who wants to build the docs locally. 

**Note**:  Read on if you don't already have an access token you can use. 

To create a github oauth token, you will need to have a registered github application. Once you have an application, you can then call 
   
    curl -i -u <gh-user> -d @gh_auth https://api.github.com/authorizations

This will prompt for your gh password using basic authentication. `gh_auth` is a local file containing: 

```JSON
{
  "scopes": ["public_repo"],
  "client_id":"<application client id>",
  "client_secret":"<application client secret>",
  "note": "For Julia Metadata Listings"
}
```
Registered applications and authorised token are visible within your github profile page. 

Contact me privately if you wan to use my registered app to create a token. 

To reiterate, you dont have to do all this to check out how the docs look. Just run `make html`, and you'll get a listing valid as of this writing.