Use cdn.rawgit.com for iconUrl instead of raw.githubusercontent.com.

This was suggested here: https://github.com/chocolatey/chocolatey-coreteampackages/issues/15
Merge pull request #13 from TomOne/rawgit-iconurl

Use cdn.rawgit.com for iconUrl instead of raw.githubusercontent.com.
Use __REPLACE__ in <title>, <summary> and <description> instead of __NAME__
Add comment which propmts maintainers to read the CreatePackages wiki page

https://github.com/chocolatey/chocolatey/wiki/CreatePackages
Merge pull request #14 from TomOne/content-improvements

Use __REPLACE__ for <title>, <summary> and <description>, add comment about reading the CreatePackages wiki page