Cleaning up some old issues

[foreverallama]

I think we can clean up some old issues. Listing some here. These aren't bugs but more like feature requests.

1. Changing Compression level in HDF5 #165
2. Using matread from data in memory instead of a MAT-file #42 #96
3. Preserve order of struct fields using OrderedDict #159
4. Opening a MAT-file with both read-write access #109
5. Slicing complex matrix in HDF5 MAT-file #76

[matthijscox]

The OrderedDict is still on my mind ever since I learned the MatlabClassObject writing requires the fields to be in order. So it might actually be a good idea to make OrderedDict the default (while perhaps making the output dicttype a user option?). Then we have better read-write consistency of structs and classes.