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/tests/auditwheels.py

@@ -0,0 +1,13 @@
+
+import os
+import sys
+import subprocess
+
+
+if __name__ == "__main__":
+    assert len(sys.argv) == 2
+    workspacedir = sys.argv[1]
+    for relpath in os.listdir(workspacedir):
+        if relpath.startswith("deltachat"):
+            p = os.path.join(workspacedir, relpath)
+            subprocess.check_call(["auditwheel", "repair", p, "-w", workspacedir])