Consider better shortcut versions for getDescriptor and getObservable

[medikoo]

/cc @kamsi @mtuchowski

Currently there's $ prefix for descriptors and _ for observables.

Both seem controversial, and to uninitiated developer they provide just wrong clues on what's behind (e.g. $ may suggest some jQuery call, while _ a private property).

Initial proposal is to introduce d and o methods, So instead of obj.$foo or obj._foo, developer may use obj.d('foo') and obj.o('foo').

Better proposals are welcome