change the RestApi to make it easier to use

the old library for matching against some URL and request data was full
of strange code, and almost enforced the user to pattern match against a
list.
Now a RestApi gets a request as input data and there are some extractor
that may be used (or not) be Rest Api developers to pattern match
against the path and/or query parts.

For example, one can write:
  POST {
    case path"$paperId/compile" =>
      ???
    case req @ path"$paperId/compile" ? query"a=${int(p1)}&b=$p2" =>
      ???
    case req @ path"$paperId/q" =>
      ???
    case req =>
      val path = req.uriPath
      val query = req.query
      val accepts = req.header("Accepts")
      ???
  }

As can be seen in the last example, does not use the string
interpolators or extractors at all, everybody is free to redefine them
as they wish.

The interpolators and extractors provided by the RestApi trait are:
 - `long` that extracts a string to a long
 - `int` that extracts a string to an int
 - `date` that extracts a string with format
   "yyyy-MM-dd'T'HH:mm:ss.SSSZ" to a Date object
 - `?` that extract an `HReqData` to a pair of string representing
   respectively the path and the (non empty) query string
 - `path` the interpolator extractor that allows people to extract path
   fragments from either a string or an `HReqData?`
 - `query` the interpolator extractor that allows people to extract
   query parameters from the query string
merge common and http module in a core module

let's be realistic, what are the core features of a \BlueLaTeX server? A
synchronization server providing an HTTP Api.
Once this is stated and admitted by everybody, we can answer this new
question: why should we separate the HTTP module from the common module?
The answer is quite simple: No reason to that.

The 'look, we separate responsibilities and features in a really clean
way' argument does not hold very long when confronted to real-world
application maintenability.

This is symptomatic of this disease called 'overmodularization' that I
got when starting the new \BlueLaTeX version with OSGi. However I treated
myself and I am well again. It is never too late to fix problems and
thus both modules are now merged into one 'core' module which name is
really explicit. If you want to extend \BlueLaTeX, we build a module
using core features, that's it.

I am pretty sure future maintainers/developers will thank me for that
decision to reduce the number of modules to its strict minimum.

  Ikea / furnishing = OSGi / software: They allow to have very modular
  things, but trying to put them together generally drives you crazy
  -- Mario Fusco