tox.ini: Test with nosetests instead of ./test and measure coverage

Fixes GH-6
Make Travis CI use nosetests instead of ./test; Add stub tests so we see
coverage gaps.
Add 'coverage' to requirements.txt so we can see coverage reports in
Travis CI
Add test test_exception_HTTPBadGateway - tests when a socket.error
causes proxy_exact_request to raise an HTTPBadGateway exception
Add --cover-erase option to nosetests commands in tox.ini and
.travis.yml
tests/test_middleware.py: Test some more things; get test coverage up
from 12% to 47%.
wsgiproxy/sampleapp.py: Fix TypeError uncovered by test (using old version
of PasteDeploy?):

marca@marca-mac:~/dev/git-repos/WSGIProxy$ .tox/py26/bin/nosetests -s -v --with-coverage --cover-package=wsgiproxy --cover-erase
...
======================================================================
ERROR: test_blank_environ (tests.test_middleware.WSGIProxyMiddlewareTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/WSGIProxy/tests/test_middleware.py", line 17, in test_blank_environ
    app(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py", line 118, in __call__
    return self.application(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/sampleapp.py", line 9, in application
    greeting = config['greeting']
TypeError: 'module' object is unsubscriptable