tools: Add pep8 linter.

This commit adds the tools/lint script to run the pep8 linter
as well as modifies .travis.yml to run the linter in Travis.

tools/lint

@@ -0,0 +1,36 @@
+#! /usr/bin/env python
+
+from pep8 import check_pep8
+from server_lib import lister
+
+import sys
+import optparse
+from typing import cast, Callable, Dict, Iterator, List
+
+EXCLUDED_FILES = [
+    # This is an external file that doesn't comply with our codestyle
+    'integrations/perforce/git_p4.py',
+]
+
+def lint_all(args, options):
+
+    by_lang = cast(Dict[str, List[str]],
+                   lister.list_files(args, modified_only=options.modified,
+                                     ftypes=['py', 'sh', 'js', 'pp', 'css', 'handlebars',
+                                             'html', 'json', 'md', 'txt', 'text', 'yaml'],
+                                     use_shebang=True, group_by_ftype=True, exclude=EXCLUDED_FILES))
+    failed = check_pep8(by_lang['py'])
+    return failed
+
+def run():
+    # type: () -> None
+    parser = optparse.OptionParser()
+    parser.add_option('--modified', '-m',
+                      action='store_true',
+                      help='Only check modified files')
+    (options, args) = parser.parse_args()
+    failed = lint_all(args, options)
+    sys.exit(1 if failed else 0)
+
+if __name__ == '__main__':
+    run()