add flake8 test

tox.ini

 [tox]
-envlist = py{26,27,33,34,35,36}-WTForms{1,2}
+envlist = py{26,27,33,34,35,36}-WTForms{1,2},flake8
 skipsdist = true
 skip_missing_interpreters = true
 
+[flake8]
+max_line_length = 120
+ignore = E402
+
 [testenv]
 usedevelop = true
 deps =
@@ -11,3 +15,7 @@ deps =
     -r{toxinidir}/requirements-dev.txt
 commands =
     nosetests flask_admin/tests --with-coverage --cover-erase --cover-inclusive
+
+[testenv:flake8]
+deps = flake8
+commands = flake8 flask_admin