handle added/removed schema files and _config.yml changes for increme…

[woodpig07]

Work for what were left out in my previous PR https://github.com/Wiredcraft/jekyllpro-cms/pull/68

• Check for config file (_config.yml) and update accordingly.
• If there are new or removed schema files, re-scan related folder tree and add/remove collection files to/from index.

[woodpig07]

The second commit was to refactor and improve getFreshIndexFromGithub() with bluebird