Point xml-rs to git

Closes https://github.com/bjz/gl-rs/issues/241
Merge pull request #242 from bvssvni/xmlfix

Point xml-rs to git
Use BTreeSet instead of TreeSet.
Merge pull request #243 from serpis/use_btreeset

Use BTreeSet instead of TreeSet.
fixed for updates to rust and xml-rs
Update for xml-rs refactoring
Update for new rust (opt-in Copy and Task -> Thread)
Merge remote-tracking branch 'HeroesGrave/master' into update-rustc

Conflicts:
	src/gl_generator/registry.rs
Fix std::slice::Items -> std::slice::Iter
Merge pull request #249 from tomaka/update-rustc

Update for rustc
Generate strings instead of AST
Pass writers instead of generating strings