Many fixtures have similar code for writing the pid & ports files or
for handling HTTP requests. This commit changes the current
ExampleFixture in order to make it an abstract BaseHttpFixture class
that can be extended for specific testing purposes. This class provides
some simple Request and Response classes to represent the HTTP requests
and responses and takes care of writing the pid and ports files and
starting the HTTP server. It allows to remove lot of duplicated code
and forbidden APIs invocations.

The current AmazonS3Fixture, GoogleCloudStorageFixture, AzureStorageFixture,
and URLFixture have been removed. The AmazonS3TestServer, ,
AzureStorageTestServer, GoogleCloudStorageTestServer have been renamed
and now extend BaseHttpFixture.

This commit also fixes an issue with the AntFixture that was expecting
a false result for the waitCondition(), whereas it should expect `true`.