allow customization of http(s) port for webui

It is potentially desirable to run the splunkweb on an alternative
port. For example, if the `splunk` service should be run as a non-root
user.

Note: This doesn't implement capability of running as a non-root user.
resolve foodcritic findings for specs
add web_port to readme
Merge pull request #13 from opscode-cookbooks/allow-web-alternate-port

allow customization of http(s) port for webui (thanks jtimberman!)