Make MatlabStructArray an AbstractDict

For better consistency with MatlabClassObject and other structs/objects, and backwards compatibility with previous MAT versions, we could consider making the MatlabStructArray <: AbstractDict. I think most dict interfaces are already implemented, like key-value iteration, getindex and setindex.

I'm consider what the implications are here:

• it's actually an AbstractDict{String, Array{Any,N}}, not AbstractDict{String,Any} like all our other types
• could we still consider it an AbstractArray, an interface I still want to implement someday so that s[1,2] returns a Dict
• we should add all abstractdict interfaces (what is the abstractdict interface anyway?)
  • need to add keys/names s["new"] = arr
  • delete keys: delete!(s, "old")
  • Unless it's a class array, then it should be immutable, since the class properties are fixed?

Start by locating the MatlabStructArray implementation and comparing it with MatlabClassObject and the other struct/object types. Review Julia's AbstractDict interface alongside the existing key-value iteration, getindex, and setindex methods, then resolve the AbstractArray, class-array immutability, and type-parameter questions. Done means the supported interfaces and compatibility implications are clearly decided.