Feature: download configuration support

FOR EARLY REVIEW ONLY PLEASE
---
Allows one to customize the download URLs to be used for installing entities (see https://github.com/brooklyncentral/brooklyn/issues/525).

In particular, see interface and javadoc in:
* api/src/main/java/brooklyn/entity/drivers/DownloadsRegistry.java
* core/src/main/java/brooklyn/entity/drivers/DownloadPropertiesResolver.java

Remaining work:
* We want to be able to customize entity "add-ons" like nginx's download URL for nginx-sticky-module-1.0.tar.gz and pcre-8.32.tar.gz. This will require a chance to the DownloadsRegistry interface...