Aws::Errors::ServiceError now has a #context attribute.

The `#context` returns the RequestContext object for the
request that caused the service to return the error.
Updated ElasticTranscoder API to latest version.
Updated apis/source/ -- all models now provide their XML namespace.
Updated css in API docs.  Symbols in reference examples now have a constrasting color.
Added a GEM_ROOT constant.  This is used for loading bundled APIs.
Updated how load paths are managed.
Syntax highlighting in API docs now uses the same red for symbols and labels.
Both sets of unit tests now run back to back before integration tests.