add coverage test

Author: caimaoy

.travis.yml

@@ -1,7 +1,11 @@
 language: python
 sudo: false
 python:
-  - "2.7"
+    - "2.7"
 install:
-  - pip install -r requirements.txt
-script: nosetests
+    - pip install -r requirements.txt
+    - pip isntall coverall
+script:
+    - nosetests
+    - coverage run --source=test_mini_spider.py
+