Enable metadata mapping for identification attributes. fixes #1202
Add a lock around the overrideen `error` accessor on `RKHTTPRequestOperation`. refs #1193
Rework metadata mapping to fix breakage under OS X
Bump version to v0.20.0-rc1
Update projects for Xcode 4.6 and enable CLANG_WARN_ENUM_CONVERSION
Merge branch 'release/v0.20.0-rc1' into development
Merge branch 'development' into zerista