Move DummyHttpServer to different source

Don't want to include it in the distribution jar.
Add upload mirror test HttpServer
Cosmetics
Update POM versions
Delete most of the useless text files and added CHANGES file
Update README
Switch over to Gradle as a build system

* Delete maven build file
* Create gradle build and settings files
* Update `.gitignore`
Move files between packages

* Cosmetic changes on connection and connection factories
* Added first Spock spec, for ConnectionPool
Delete constructor for ConnectionPool
Update idea configuration on gradle plugin

* Create runners for 'specs', 'integration tests' and 'all tests'
* Set output build path to `build/idea` instead of `out` (and update .gitignore file)
* Configure default JDK and language level to Java 7
Fix inverted check on AbstractHttpClient's setters
Rename request inactivity timeout setter input parameter
Delete unused HttpConnectionFactory method
Cosmetic changes on DefaultHttpRequestFuture and its factory
Increase spec coverage for ConnectionPool
Add specs for both HttpConnection factories
Rename HttpRequestFutureFactory's getFuture to createFuture

Also deleted the unused no-argument variant.
Simply HttpRequestFuture by discarding optional cancellable property

All futures in the context of this project are cancellable so there's no point in exposing an optional property.
Promote attachConnection() to interface method

Better than the alternative of assuming all futures to be of the only kind that exposed this as a public method. This needs to be reviewed later.
Cosmetics and import optimizations