Add ability to execute XML-RPC calls other than "Bug.xxx"

Now you can execute arbitrary API methods instead of only ones on the Bug namespace.

```ruby
ActiveBugzilla::Base.service.execute(command, params)
```

I also added a convenience method on the main module, so that if `Base.service` is set, we can do stuff like:

```ruby
ActiveBugzilla.execute(command, params)
```
or, equivalently...
```ruby
ActiveBugzilla.call(command, params)
```

See issue #44