Struct array writing

[matthijscox]

My attempt to solve #206 and allow struct array writing in HDF5 mat files.

Changes:

• defines m_write for AbstractArray{<:AbstractDict} as interface for struct arrays
• refactor cell array writing (m_write for AbstractArray{T}) to have common reference writing
• added readme example

[matthijscox]

Defined the MatlabStructArray type to enabled read-write consistency of struct arrays for v7.3 mat files. I'm trying to keep the type as backwards compatible as possible with the previous Dict{String,Any} type that contains arrays in the field, yet also allow easy conversion to arrays of dicts which is similar to cell array behavior.

I don't know yet how to distinguish this struct array in mat versions before v7.3.