Improved build.xml - cleaner, reduced build time in most cases

This makes building BC a two-step process:

* Clone repo
* Run `ant`

Now builds in 'build' directory inside the repo dir - prevents jenkins workspace messiness.  
No longer unnecessarily checks for updates to MCP if the file already exists - MCP doesn't appear to get silent updates, version number always changes.  
Only sets up forge+mcp if forge or MCP have updated, else merely recompiles and reobfuscates.  
Localisations are still in the parent directory - this should probably change in the future.  
Build number is now set to "unknown" when not set as an environment variable by jenkins/whatever. This helps prevent unofficial builds with the same filename as an official build from floating around.  
The commons lib is no longer downloaded - it does not seem to be necessary.

Signed-off-by: Ross Allan <rallanpcl@gmail.com>