Update configuration

To implement #39 we need a way to configure the prefix of the \BlueLaTeX
Rest API.
The configuration was reworked to be able to bind to any host as well.
[imp #39] Make the API prefix configurable

If \BlueLaTeX is not installed at the root of the virtual host, using
the API matchers strips the configured prefix for you, so you can write
your paths without worrying about it.
[imp #39] Add application configuration

The client may request the public configuration data that indicates
(amongst other things) the prefix of the Rest API, and the ReCaptcha
public key.
This object may actually contain any data needed by the client to be
successfully started.
Merge pull request #71 from satabin/wip-not-root

Wip not root