Update to Jekyll 3

[hexacyanide]

This pull request accomplishes the following:

• remove unmaintained Git metadata plugin as dependency
• add a replacement for the above that has no dependencies
• update from Jekyll 2.5 to 3.1.1

Since Jekyll 3 now generates static pages by creating an index.html inside a directory, the following changes were also required:

• link extensions were dropped (e.g. login.html -> login)
• asset references were changed to be absolute

[asayler]

Thanks! I'll shoot to merge this by early next week.