attempting to fix link to valid options properties

Current link for 'valid properties are available here' goes to url:

https://github.com/nodejitsu/node-http-proxy/blob/caronte/tree/master/lib/http-proxy.js#L26-L39

The url works fine if 'tree/master/' is removed, so this is trying to remove that part of the relative path.

The same removal of 'tree/master/' is being made for the 'available here' link that is preceded by "When a request is proxied it follows two different pipelines" since it suffers the same issue.
Merge pull request #502 from jamesmanning/patch-1

attempting to fix links to 2 source locations in README.md