replace travis with circle-ci, cleanup of build steps (#639)

Travis is getting slower and was already hard to debug. This commit: 

- moves from travis to Circle-CI
- has a ci-scripts/README.md which explains what is contained there
- puts all build and CI scripts into "ci_scripts" directory
- allows to run full ci runs locally
- trims down the base Docker image where tests/docs/wheels are run from 4.4GB to 1.7GB
- fixes #628

Using circle-ci also means that we can probably integrate Mac-OS build steps more easily.

I am sure that my initial circle-ci config (helped by kind hints from @dignifiedquire ) can be improved.

python/doc/api.rst

@@ -10,9 +10,9 @@ high level API reference
   other classes)
 - :class:`deltachat.chatting.Contact`
 - :class:`deltachat.chatting.Chat`
-- :class:`deltachat.chatting.Message`
-- :class:`deltachat.chatting.MessageType`
-- :class:`deltachat.chatting.MessageState`
+- :class:`deltachat.message.Message`
+- :class:`deltachat.message.MessageType`
+- :class:`deltachat.message.MessageState`
 
 Account
 -------
@@ -36,19 +36,19 @@ Chat
 Message
 -------
 
-.. autoclass:: deltachat.chatting.Message
+.. autoclass:: deltachat.message.Message
     :members:
 
 MessageType
 ------------
 
-.. autoclass:: deltachat.chatting.MessageType
+.. autoclass:: deltachat.message.MessageType
     :members:
 
 MessageState
 ------------
 
-.. autoclass:: deltachat.chatting.MessageState
+.. autoclass:: deltachat.message.MessageState
     :members: