Add an option to download a jar instead of doing a gradle build

I'd rather not perform a gradle build on the node. This patch adds the ability to pull down a precompiled exhibitor jar via the web and skip the build step. (Default is set to build to keep the current behavior). 

New attributes added:

```
default[:exhibitor][:install_method] = 'build'
default[:exhibitor][:mirror] = 'http://example.com/exhibitor-1.5.0.jar'
default[:exhibitor][:checksum] = ''
```