Add vectorized string methods to Series
I'd appreciate any feedback on the API (@takluyver and others), but this makes a lot of sense to me and I'm already finding my fingers falling right onto it. I swear I'm going to start doing more pull requests

API looks great
 Hm. Should maybe raise an exception in repeat on numeric data, but how to do so without paying a high performance penalty...
 Maybe a hack for now? f0afc3ba48
 Nice, that works for me. Add a test for that and good to go?
 yeah, I'm putting in a few more changes to handle unicode where the method was constructed with str.\* methods right now. Will push in a little bit
 Thanks for the feedback. Merged up and closing relevant issues now
