Nightlies have changed to_str -> to_string

This is with:

rustc 0.11.0-nightly (1c711db551b9a5e56ba76e002e287dcfbb2cff3c 2014-07-09 00:36:40 +0000)

I assume it's going to be a breaking change, so here it is.
Merge pull request #109 from pcn/fix-to-str-to-string

Nightlies have changed to_str -> to_string