[Echo.Loader] re-structure test canvases files & appropriate fixes
[Echo.Loader][Echo.Canvas][Tests] CR fixes:
 -remove undefined values from the default Canvas config;
 -move "_error" method to the Canvas class;
 -retrieve canvas specific data from the jQuery object data with the unified namespace "data-canvas-*";
 -fix typos and comments for test assertions.

[Tests] re-structure test canvas files.

[Echo.Canvas] rename marker which indicates that canvas was initialized at specific jQuery object (initialized -> echo-canvas-initialized).

[Echo.API.*] fix "available" static method for the XDomainRequest transport object