promoted ann.id and enabled id as a class property of ann

[bohJiang12]

Update

This commit aims to fix issue #296 which expects following:

1. Current MMIF 1.x uses long-form of annotation's id (e.g. instead of ann.id, rather view.id:ann.id across the whole MMIF world
2. Promote the annotation's id which originally resides in AnnotationProperties class to the Annotation class and make it as a class property as View class did to its id.

Discussion

One issue was found while implementing, that is, the location of annotation's id in raw MMIF JSON file (the input/output source file for MMIF SDK) mismatches our expected location to it. The cause of such mismatch is that the annotation's id would naturally be put under AnnotationProperties class by default and uses the short-form id as is during deserialization. Therefore, it's not possible to achieve the first goal stated above.

Then, one feasible approach to target it is that we need to implement a "converter" utility as an interface between raw JSON MMIF and the SDK so that the raw JSON file can be converted to a desired format for the SDK. One advantage of the approach is that it requires minimal change for the SDK and it is isolated to the SDK.