For development statically link dcc-rs

This links the python bindings statically to libdeltachat.a if the
DCC_RS_DEV environment variable is set to the project's root.  This is
a little simpler then requiring the manual CFLAGS and LD_LIBRARY_PATH
tweaking.

It also adds a script to easily invoke the integration tests locally
without forgetting steps.

python/tox.ini

@@ -10,15 +10,17 @@ envlist =
 commands = 
     pytest -rsXx {posargs:tests}
     pip wheel . -w {toxworkdir}/wheelhouse
- 
 passenv = 
     TRAVIS 
-    LD_LIBRARY_PATH 
-    CFLAGS
+    DCC_RS_DEV
 deps = 
     pytest
     pytest-faulthandler
     pdbpp
+    cffi
+    requests
+    attrs
+    six
 
 [testenv:auditwheels]
 skipsdist = True