RequestManager: Move base_url prepending to json_req()

Is better to do the prepending as soon as possible, so lower level
methods stay as abstract as possible (and can be reused for other
purposes).
RequestManager: Change HTTP methods to uppercase
RequestManager: Add a method to get HTTP OPTIONS

This is not really used, is just for completeness.
RequestManager: Don't use `config` directly

Make RequestManager more agnostic to the current usage so is easier to
reuse.
Fix #61: Parse Link HTTP headers for full retrieval

When querying large lists to GitHub, it replies with a "Link:" header
pointing to links to the next "page" of data. This commit parses this
header and automatically retrieves the next page until no next page is
found.