Specify role for deployment tasks

It's really hard to perform multi-server deployments with thinking-sphinx.
Everything get's executed on all servers even though sphinx is only running on
one of them. This allows to do the following:

set :thinking_sphinx_roles, :search
require 'thinking_sphinx/capistrano'

server "search.example.com", :rails_app, :search
Merge pull request #533 from AndreyChernyh/multi-server-deploys

Specify role for deployment tasks