Better dependency downloader

Hi,

I wasn't really happy with the way the H5P-CLI tool downloads dependencies of libraries, so I created a new small application, which has several improvements over the somewhat outdated CLI tool:

• The H5P CLI uses a static library information file. This means that changes in the dependencies of libraries might not be reflected. This application gets the dependencies directly from the library.
• The library information used by the H5P CLI is out of date and can't be updated locally. This application adds its own more up-to-date list and tries to guess the location of unknown libraries on GitHub.
• The H5P CLI tool does not download dependencies for the editor or libraries that are used in semantics.json but not in library.json.
• The H5P CLI tool forces you to have GitHub account and to add your SSH key to GitHub.

You can find the application here on GitHub. 

Sebastian