Fix for Issue 75

This does a couple of things in relation to Issue 75 - both were ones we wanted to do:

* `NexusAnalyzer` will now use the proxy if it's configured *and* `analyzer.nexus.proxy` is `true` (it's configured to `true` in the default `dependencycheck.properties`)
* A preflight request is added which will check the status of the Nexus repository. If this preflight request fails, the Nexus analyzer will be disabled

This solves an open challenge we had where the user might configure a proxy but might also be using an internal Nexus repository where they might not want to use the proxy. The out-of-the-box configuration is the most appropriate one, and others work somewhat elegantly:

* I'm not behind a proxy, and I want to use Central - a proxy won't be used (because the proxy settings are not turned on, even though `analyzer.nexus.proxy` is `true`)
* I configure a different Nexus URL, but don't set a proxy - as above
* I set a proxy but don't have an internal proxy - because the proxy settings are enabled and the default for `analyzer.nexus.proxy` is `true`, we'll use the proxy and use it to connect to Central
* I set a proxy, but have an internal Nexus repository. If I go through the trouble of configuring that URL in the properties, the appropriate property to disable the proxy for Nexus requests is right there.