Introduced WebServerSupplierFactory,
@Machine allowing to gather available WebServerSuppliers in SPI
Provided jetty/tomcat/simple web server suppliers
in their own modules
Removed hard dependency from restx-spec-tests to restx-server-jetty
Keeping it for unit tests in this module.
The restx-spec-tests user will have the choice to use the server impl of its choice
simply by adding the restx-server impl in the classpath.

BACKWARD COMPAT breaks out here : some RestxSpecRule & RestxSpecTestServer
disappear since they were too much related to a restx-server-jetty impl
In order to fix it, in most of cases, using the no-args constructor (relying on default
WebServerSupplier config) or constructor with a WebServerSupplier (using the one provided in
your restx-server artefact) would fix the issue.
Created new module restx-servers-testing,
allowing to provide a unique entry point for generic restx-server scenarii

Note that we don't have the choice to create a dedicated module for this,
because otherwise, maven will complain with circular dependencies between
restx-specs-test -(test)-> restx-server-jetty -(test)-> restx-specs-test
Made some naming refactorings in restx-server modules
fix NPE due to factory not being bound on query
Deleted useless WebServerSupplierFactory
Merge pull request #32 from fcamblor/webserver-supplier-refacto

Removing hard dependency from restx-specs-test to restx-server-jetty

The restx-specs-test rules will then rely either on `@Procuced WebServerSupplier` (if no `WebServerSupplier` is passed as a constructor arg to the `@Rule`)

Important note : this introduced a backward incompat in restx-spects-test `@Rule` constructors, too much tied to the restx-server-jetty implementation, which was removed.

To fix this, you will either have :
- To use simple default constructor which will rely on default web server config available in classpath
- To provide a WebServerSupplier by hand if you want to override default web server config

Another backward compat is `ServerSuppliers` utility class doesn't exist anymore: `WebServerSupplier` can be retrieved directly from the restx-server-* utilities (for instance, `JettyWebServer.jettyWebServerSupplier()`)