Shuffle http related test code. Hopefully it ends up at the right place :)

* bzrlib/tests/HttpServer.py: 
New file. bzrlib.tests.ChrootedTestCase use HttpServer. So the
class can't be defined in bzrlib.tests.HTTPUtils because it
creates a circular dependency (bzrlib.tests.HTTPUtils needs to
import bzrlib.tests).

* bzrlib/transport/http/_urllib.py: 
Transfer test server definition to bzrlib.tests.HttpServer. Clean
up imports.

* bzrlib/transport/http/_pycurl.py: 
Transfer test server definition to bzrlib.tests.HttpServer. Clean
up imports.

* bzrlib/transport/http/__init__.py: 
Transfer all test related code to either bzrlib.tests.HttpServer
and bzrlib.tests.HTTPUtils.
Fix all use of TransportNotPossible and InvalidURL by prefixing it
by 'errors.' (this seems to be the preferred way in the rest of
bzr).
Get rid of unused imports.

* bzrlib/tests/test_transport.py:
(ReadonlyDecoratorTransportTest.test_local_parameters,
FakeNFSDecoratorTests.test_http_parameters): Use HttpServer from
bzrlib.tests.HttpServer instead of bzrlib.transport.http.

* bzrlib/tests/test_sftp_transport.py:
(set_test_transport_to_sftp): Use HttpServer from
bzrlib.tests.HttpServer instead of bzrlib.transport.http.

* bzrlib/tests/test_selftest.py:
(TestTestCaseWithTransport.test_get_readonly_url_http): Use
HttpServer from bzrlib.tests.HttpServer instead of
bzrlib.transport.http.

* bzrlib/tests/test_repository.py: 
Does *not* use HttpServer.

* bzrlib/tests/test_http.py: 
Build on top of bzrlib.tests.HttpServer and bzrlib.tests.HTTPUtils
instead of bzrlib.transport.http.

* bzrlib/tests/test_bzrdir.py:
(ChrootedTests.setUp): Use HttpServer from bzrlib.tests.HttpServer
instead of bzrlib.transport.http.

* bzrlib/tests/branch_implementations/test_http.py:
(HTTPBranchTests.setUp): Use HttpServer from bzrlib.tests.HttpServer
instead of bzrlib.transport.http.

* bzrlib/tests/branch_implementations/test_branch.py:
(ChrootedTests.setUp): Use HttpServer from bzrlib.tests.HttpServer
instead of bzrlib.transport.http.

* bzrlib/tests/__init__.py:
(ChrootedTestCase.setUp): Use HttpServer from
bzrlib.tests.HttpServer instead of bzrlib.transport.http.