Support Webdriver custom options by kwargs

Can you run the tests and see if it breaks anything, seems there is some problem with running tests on my pc.

This is the error I see when I run tests:
```
platform linux2 -- Python 2.7.4 -- pytest-2.3.5
collected 1 items / 1 errors 

scrapy_webdriver/tests/test_request_queue.py .

=========================================================== ERRORS ============================================================
___________________________ ERROR collecting build/lib/scrapy_webdriver/tests/test_request_queue.py ___________________________
import file mismatch:
imported module 'scrapy_webdriver.tests.test_request_queue' has this __file__ attribute:
  /home/samos/workspace/scrapy-webdriver/scrapy_webdriver/tests/test_request_queue.py
which is not the same as the test file we want to collect:
  /home/samos/workspace/scrapy-webdriver/build/lib/scrapy_webdriver/tests/test_request_queue.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
============================================= 1 passed, 1 error in 93.28 seconds ===========
```