Run flake8 on Travis

.travis.yml

@@ -0,0 +1,12 @@
+sudo: false
+dist: trusty
+language: python
+python:
+  - "2.7"
+  - "3.4"
+  - "3.5"
+  - "3.6"
+install:
+  - pip install -r requirements.txt
+script:
+  - flake8

requirements.txt

@@ -1,2 +1,3 @@
+flake8==3.3.0
 sphinx
-alabaster
+alabaster

setup.cfg

@@ -0,0 +1,10 @@
+[flake8]
+exclude = .git,__pycache__,docs/
+ignore =
+    # The default ignore list:
+    E121,E123,E126,E226,E24,E704,
+    # Our additions:
+    # E127: continuation line over-indented for visual indent
+    # E128: continuation line under-indented for visual indent
+    # E501: line too long
+    E127,E128,E501