Add HTTP-to-CD mirroring

[trustin]

Motivation:

It would be very useful to set up a simple mirror that mirrors the result of an HTTP query into a repository, just like IFTTT.

Modifications:

• Added support for the following two new URL schemes:
  • http
  • https
• Renamed Mirror.remotePath to remoteSubpath to better reflect what it actually does.
• Made Mirror.remoteSubpath nullable because HTTP-to-CD mirror doesn't need it.
• Added HttpMirror and HttpMirrorTest.

Result:

• A user can mirror a certain HTTP endpoint into a file in Central Dogma.
• Closes #154

Future works:

• Add support for basic and token-based authentication
• Add documentation