SETUP move code to src

This way we can ensure that the packaged code is tested.
See https://hynek.me/articles/testing-packaging/ or c62a78c015

+ DOC adapt doc to new structure and test doc with travis

doc/conf.py

@@ -20,7 +20,7 @@ from setuptools_scm import get_version
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
-sys.path.insert(0, os.path.abspath('..'))
+sys.path.insert(0, os.path.abspath('../src'))
 root = os.path.relpath(os.path.join(os.path.dirname(__file__), '..'))
 
 # -- General configuration ------------------------------------------------
@@ -42,6 +42,11 @@ extensions = [
     'sphinx.ext.inheritance_diagram',
 ]
 
+# supress warnings for external images
+suppress_warnings = [
+    'image.nonlocal_uri',
+]
+
 # enable todos
 todo_include_todos = True