Avoid boxing in Skin dictionary.

closes #156
changed texture loaded to pass the current context.
Merge pull request #158 from agmcleod/master

changed texture loaded to pass the current context.
Refactored AttachmentLoader, allowing parameters per attachment type. Added attachment path. Added per attachment color.
Merge branch 'origin/master'
Fixed bounding boxes when slot has no attachment.
Merge branch 'master' of https://github.com/EsotericSoftware/spine-runtimes