Improved support for obj properties and arrays dimentions

Added support for defining default obj properties, assigning obj
properties and assigning array dimensions
Specialized assign dim

Adds support for appending to array ($array[] =), and creating array by
assigning to it ($a[] = or $a[0] = when $a is not set or null).
Merge pull request #13 from arnaud-lb/assign-dim

Specialized assign dim