Update to rust version 0.13.0-nightly (8bca470c5 2014-12-08 00:12:30 +0000):
std::str::MaybeOwned -> std::str::CowString
Update to rust version 0.13.0-nightly (8bca470c5 2014-12-08 00:12:30 +0000):
json -> Json
Use std even in for_c builds

See #63.
Fix for_c::common for Rust upgrade
More macro cleanup
Merge branch 'pr62'