[core] Break up ddtrace.settings into sub-modules (#814)

* [core] Break up ddtrace.settings into sub-modules

* Add back the default global config

* Fix import name

Author: brettlangdon

ddtrace/settings.py

@@ -0,0 +1,17 @@
+from .config import Config
+from .exceptions import ConfigException
+from .http import HttpConfig
+from .hooks import Hooks
+from .integration import IntegrationConfig
+
+# Default global config
+config = Config()
+
+__all__ = [
+    'config',
+    'Config',
+    'ConfigException',
+    'HttpConfig',
+    'Hooks',
+    'IntegrationConfig',
+]