Add integration tests.

.gitignore

 __pycache__/
 *.py[cod]
+tests.integration.*.log
+tests.integration.*.png
 vectordraw_xblock.egg-info/
 var/

test-requirements.txt

 -r requirements.txt
 -e git+https://github.com/edx/xblock-sdk.git@629650ed6fc1b77d1a5287debeace0b29db7c0fd#egg=xblock-sdk
+ddt

tests/integration/xml/custom.xml

+<vertical_demo>
+  <vectordraw url_name="vectordraw_example"
+              display_name="Custom Exercise"
+              description="Custom exercise description"
+              width="600"
+              height="450"
+              axis="true"
+              show_navigation="true"
+              show_vector_properties="{{ show_vector_properties }}"
+              show_slope_for_lines="true"
+              add_vector_label="Custom button label"
+              vector_properties_label="Custom properties label"
+              background_url="https://github.com/open-craft/jsinput-vectordraw/raw/master/Notes_and_Examples/2_boxIncline_multiVector/box_on_incline.png"
+              background_width="20"
+              vectors="{{ vectors }}"
+              points="{{ points }}"
+              expected_result="{{ expected_result }}"
+              />
+</vertical_demo>

tests/integration/xml/defaults.xml

+<vertical_demo>
+  <vectordraw url_name="vectordraw_example" />
+</vertical_demo>