Enable for a much more generic implementation

First of all, can I thank you for your original code, it was a great help for me to implement what I was aiming for with a dynamic deployment solution.

However I am currently using rackspace which doesn't support tagging of server instances. 

I have updated your code to enable users to pass a specific block of code, when returned true will include the server within capistrano's server list. 

I have also updated the readme.md to reflect the API changes and examples.

I l'd like to hear what your thoughts are regarding this? and/or if you would like my to implement this differently?

*This pull request breaks the existing API but I feel as though this is a much more generic implementation*