Fixed YAMLLoader (1.1b and 1.0b)

Accidentally added v1.1 into 1.0 4h ago.
1.0b is a slightly updated 1.0 compliant version.
1.1b has added an exposed MessageHelper class inside YAMLLoader to
enable "low-level" access to messages when appropriate (enables creating
custom code with consistent messaging behavior) ... (this will be used
in StringLoader in the next commit).
StringLoader 1.0b and 1.1

minor update to 1.0 (now throwing on plugin !enabled similar to
YAMLLoader)
medium update to 1.1 to make use of the added features in YAMLLoader
1.1b to eliminate (near)duplicate code in StringLoader