Fixed "Private type in exported signature" issue.

Building on recent builds of the compiler throws:
```
branchify.rs:25:73: 25:84 error: private type in exported type signature
branchify.rs:25 pub fn branchify(options: &[(&str, &str)], case_sensitive: bool) -> Vec<ParseBranch> {
                                                                                        ^~~~~~~~~~~
branchify.rs:87:21: 87:32 error: private type in exported type signature
branchify.rs:87         branches: &[ParseBranch],
                                    ^~~~~~~~~~~
```
since you can't have private symbols in your public interface any more. This PR makes ParseBranch public in ```branchify.rs``` to fix this problem. Encapsulation is not lost since the fields are still private.