Add flake8 --exit-zero to the Travis CI process

Author: cclauss

.flake8

@@ -0,0 +1,40 @@
+[flake8]
+  count = True
+  # 127 characters is the width of a GitHub editor
+  max-line-length = 127
+  statistics = True
+  # E401 multiple imports on one line
+  # E402 module level import not at top of file
+  # E502 the backslash is redundant between brackets
+  # E701 multiple statements on one line (colon)
+  # E703 statement ends with a semicolon
+  # E711 comparison to None should be 'if cond is not None:'
+  # E712 comparison to False should be 'if cond is False:' or 'if not cond:'
+  # E713 test for membership should be 'not in'
+  # E731 do not assign a lambda expression, use a def
+  # E999 SyntaxError: invalid syntax
+  # F401 'time' imported but unused
+  # F403 'from math import *' used; unable to detect undefined names
+  # F405 log may be undefined, or defined from star imports: math
+  # F811 redefinition of unused 'radians' from line 26
+  # F821 undefined name 'filename'
+  # F841 local variable 'e' is assigned to but never used
+  # W191 indentation contains tabs
+  select =
+    E401,
+    E402,
+    E502,
+    E701,
+    E703,
+    E711,
+    E712,
+    E713,
+    E731,
+    E999,
+    F401,
+    F403,
+    F405,
+    F811,
+    F821,
+    F841,
+    W191,

.travis.yml

@@ -5,6 +5,14 @@ python:
   - "3.3"
   - "3.5"
 
+install:
+  - pip install -r requirements.txt
+  - pip install flake8
+
+before_script:
+  # --exit-zero means that flake8 will only warn and not stop the build
+  flake8 . --exit-zero
+
 script:
   # NOTE: we must do all testing on the installed python package, not 
   # on the build tree. Otherwise the testing is invalid and may not
@@ -13,7 +21,6 @@ script:
   # Set pythonpath
 
   # install
-  - pip install -r requirements.txt
   - git clone git://github.com/mavlink/mavlink.git
   - ln -s $PWD/mavlink/message_definitions ../
   - python setup.py build install