Add ability to specify ports for wsgi test server

[mrmin123]

Same PR as #25 except the branch is now in this repo.

The proposed code modifies the wsgi_server_host_port fixture in serverfixtures.py so that if two specific arguments are passed to it, it will initiate the WSGI test server in the specified port range. This is useful if your browser tests need to be run on specific ports (eg ClinGen needs to be run on specific ports for testing Auth0 authentication). If the arguments are missing the fixture will default to webtest.http's get_free_portmethod as before.

The extra code was derived from webtest.http's get_free_port method, with modifications to make it loop through the specified port range until it finds a free port. If a free port is not found within the range, it will raise an exception.

Testing:

1. Browser tests should run on any open port (default/original behavior):
   bin/test -m bdd -v --splinter-webdriver chrome

2. Browser tests should run between ports 65525 and 65535:
   bin/test -m bdd -v --splinter-webdriver chrome --wsgi-arg port_range.min 65525 --wsgi-arg port_range.max 65535

3. Browser tests should fail due to occupied port (assuming port 80 is occupied):
   bin/test -m bdd -v --splinter-webdriver chrome --wsgi-arg port_range.min 80 --wsgi-arg port_range.max 80

4. Browser tests should run on any open port (default/original behavior because port_range.max is missing):
   bin/test -m bdd -v --splinter-webdriver chrome --wsgi-arg port_range.min 65525

For derivative projects (eg clincoded, encoded) their tests/__init.py file will need the additional line in the `pytest_addoption function for the command line arguments to be read properly:

parser.addoption('--wsgi-arg', nargs=2, dest='wsgi_args', action='append', type='string')

If the specified ports are also required for travis tests, the arguments must be added to .travis.yml as well. An example (see last two lines):

    if test -n "$BROWSER"; then
      test -s "$SC_PIDFILE" && sleep 10 && bin/test -v -v --timeout=600 -m "bdd" --tb=short \
        --splinter-implicit-wait 60 \
        --splinter-webdriver remote \
        --splinter-remote-url "http://$SAUCE_USERNAME:$SAUCE_ACCESS_KEY@localhost:4445/wd/hub" \
        --splinter-socket-timeout 600 \
        --browser-arg tunnel-identifier "$TRAVIS_JOB_NUMBER" \
        --browser-arg-int build  "$TRAVIS_BUILD_NUMBER" \
        --browser-arg-int idleTimeout 600 \
        --browser-arg name "$TRAVIS_REPO_SLUG $TRAVIS_BRANCH $TRAVIS_COMMIT" \
        --browser-arg browser "$BROWSER" \
        --wsgi-arg port_range.min 65525 \
        --wsgi-arg port_range.max 65535