Add Tox setup

#### Added nosetests and flake8 integration through tox
#### Failing flake8 are fixed in #7 

```
tox                                                                          [14:02:28]
GLOB sdist-make: /nail/home/prateek/pg/swagger-py/setup.py
py inst-nodeps: /nail/home/prateek/pg/swagger-py/.tox/dist/swaggerpy-0.3.0.zip
py runtests: commands[0] | nosetests
..................................................................
----------------------------------------------------------------------
Ran 66 tests in 0.457s

OK
py runtests: commands[1] | flake8 swaggerpy
swaggerpy/__init__.py:13:1: F401 'Loader' imported but unused
swaggerpy/__init__.py:13:1: F401 'load_file' imported but unused
swaggerpy/__init__.py:13:1: F401 'load_json' imported but unused
swaggerpy/__init__.py:13:1: F401 'load_url' imported but unused
swaggerpy/__init__.py:14:1: F401 'SwaggerError' imported but unused
swaggerpy/__init__.py:14:1: F401 'SwaggerProcessor' imported but unused
swaggerpy/swagger_model.py:88:80: E501 line too long (83 > 79 characters)
swaggerpy/swagger_model.py:314:80: E501 line too long (80 > 79 characters)
swaggerpy/swagger_model.py:332:80: E501 line too long (80 > 79 characters)
swaggerpy/swagger_model.py:427:80: E501 line too long (82 > 79 characters)
ERROR: InvocationError: '/nail/home/prateek/pg/swagger-py/.tox/py/bin/flake8 swaggerpy'
py runtests: commands[2] | flake8 swaggerpy_test
swaggerpy_test/resource_listing_test.py:50:80: E501 line too long (84 > 79 characters)
swaggerpy_test/resource_listing_test.py:56:80: E501 line too long (84 > 79 characters)
swaggerpy_test/resource_listing_test.py:63:80: E501 line too long (88 > 79 characters)
swaggerpy_test/resource_api_test.py:72:80: E501 line too long (88 > 79 characters)
swaggerpy_test/resource_model_test.py:286:80: E501 line too long (99 > 79 characters)
swaggerpy_test/resource_model_test.py:295:80: E501 line too long (99 > 79 characters)
swaggerpy_test/resource_model_test.py:304:80: E501 line too long (104 > 79 characters)
ERROR: InvocationError: '/nail/home/prateek/pg/swagger-py/.tox/py/bin/flake8 swaggerpy_test'
______________________________________________________________ summary _______________________________________________________________
ERROR:   py: commands failed
```
(for context from an IRL discussion)

If the test files are in requirements-dev.txt along with dev dependencies, running your tests now depends on dev dependencies. For example if you have ipython as a dev dependency, your code might accidentally import ipython (that won't exist in production) and your tests won't catch this error (because ipython is also installed in the test env)