Read support for MAT-file v4

[foreverallama]

• Added Read Support for MAT-file v4
• Change return type of whosmat from list(var_name, shape, classname) to dict{var_name: (shape, classname)}
• Misc: Refactor tests to include v4 format
• Misc: Update README

Some notes:

• No support for VAX-D, VAX-G and CRAY floating point formats. Do not plan to support unless requested.
• Will not support write operations to v4 format.
• Classnames returned by whosmat now mostly reflects MATLAB classnames displayed in the workspace (except for sparse, which is only returned as "sparse")

Fixes #60

[foreverallama]

Just documenting a MATLAB bug I found when working on this:

If I save a 0x0 complex double in v7.3, it loads back as a real variable. That's because MATLAB does not use any identifiers for complex data in v7.3 format, it's defined by the type of HDF5 dataset. However, since there is no data in a 0x0 array (the data is the shape of the empty variable), it loses the identifier for complex type.