Run flake8 as part of the build

For files in the python_bindings folder, run flake8 to detect style
issues and errors automatically.

.travis.yml

@@ -27,7 +27,7 @@ before_install:
   source venv/bin/activate
 install:
   - cmake similarity_search && make -j 4
-  - travis_wait travis_retry pip install -r python_bindings/requirements.txt scipy six
+  - travis_wait travis_retry pip install -r python_bindings/requirements.txt scipy six flake8
   - travis_retry cd python_bindings && python setup.py build install && cd ..
 
 script:
@@ -40,6 +40,7 @@ script:
   fi
   cd python_bindings
   python setup.py test
+  flake8
   cd ..
 cache:
   - apt

python_bindings/setup.py

@@ -129,9 +129,12 @@ def build_extensions(self):
     description='Non-Metric Space Library (NMSLIB)',
     author='Leonid Boytsov',
     url='https://github.com/searchivarius/nmslib',
-    long_description="""Non-Metric Space Library (NMSLIB) is an efficient cross-platform similarity search library and a toolkit for evaluation of similarity search methods.
-The goal of the project is to create an effective and comprehensive toolkit for searching in generic non-metric spaces. Being comprehensive is important, because no single method
-is likely to be sufficient in all cases. Also note that exact solutions are hardly efficient in high dimensions and/or non-metric spaces. Hence, the main focus is on approximate me thods.""",
+    long_description="""Non-Metric Space Library (NMSLIB) is an efficient cross-platform
+ similarity search library and a toolkit for evaluation of similarity search methods. The
+ goal of the project is to create an effective and comprehensive toolkit for searching in
+ generic non-metric spaces. Being comprehensive is important, because no single method is
+ likely to be sufficient in all cases. Also note that exact solutions are hardly efficient in
+ high dimensions and/or non-metric spaces. Hence, the main focus is on approximate methods.""",
     ext_modules=ext_modules,
     install_requires=['pybind11>=2.0', 'numpy'],
     cmdclass={'build_ext': BuildExt},

python_bindings/tests/legacy_test.py

@@ -13,6 +13,7 @@
 
 import nmslib
 
+
 class DenseTests(unittest.TestCase):
 
     def setUp(self):

python_bindings/tox.ini

@@ -5,3 +5,14 @@ envlist = py27, py35, py36
 commands = {envpython} setup.py test
 deps =
     -rrequirements.txt
+
+[testenv:style]
+deps =
+    -rrequirements.txt
+    flake8
+
+commands =
+    flake8
+
+[flake8]
+max-line-length = 100