Adding basic support for structs, cell arrays and strings

Hi, 

I modified the code to support a few more data types.
Basically, if it's a cell or a struct it will convert those to the appropriate lua tables and read the sub-fields recursively (which might be tensors or other structs or cells).
There is one breaking change (not back-compatible) related to strings. I decided to read C_CHAR to lua strings instead of torch CharTensor. For the data I have, this seems to be the adequate thing to do (matches what octave does, for example). However, I understand that other code relying on this package could break, so it might not be a good idea for everyone. Feel free to comment on that.

Cheers,
Hugo