Option to specify path prefix for when Splunk is behind a reverse proxy

It would be useful to support providing a path prefix for when Splunk is hosted behind a load balancer or reverse proxy.  e.g. Splunk may be running behind an Apache HTTPd proxy with config something like:
```
ProxyPass /test_splunk_api http://localhost:8089
```
In this scenario we could then use something like:
```ruby
Splunk::Service.new(
  :host => 'example.com',
  :path => '/test_splunk_api'
)
```
If you do not have time ATM I am happy to make this change and raise it as a pull request.