Update README.md

Make the supported versions slightly clearer
Merge pull request #22 from brendano/patch-1

Update README.md
Support "do" syntax for matopen
Hopefully fix Travis builds
Support sparse matrices in MATLAB v6/v7 format

Also:
- Fix an issue related to reading logicals from MAT v5 files
- Fix reading complex sparse matrices from MATLAB v7.3 files
- Make test output more verbose

Fixes #16
Fix some type consistency issues
Fix type consistency with logical arrays and add test
Support writing SparseMatrixCSC to MAT

Closes #24
Add missing secret hint for logical encoding
Fix reading empty strings and screwy UTF-8 strings from newer MATLAB
Fix writing empty strings
Actually write the null byte for empty strings

MATLAB doesn't seem to care, but this seems right
Initial work on reading undoc opaque objects

Matlab's opaque classes (handle classes) are stored in an undocumented manner.
This defines the class id for mxOPAQUE_OBJECT and enables reading of the
object data.
Add very basic subsystem extraction

If subsys_offset is nonzero (or not spaces, yay backwards compatibility!), then
there's an extra unnamed miMATRIX at the end of the file. This matrix contains
the data of a complete matfile (except without the header), containing all the
data for class objects in an undocumented layout.  Crazily, this subsystem can
additionally include another matrix element that contains yet another matfile.
Refactor to read the subsystem on file open

Class objects need access to the MCOS subsystem data when read, so read it first if it is there.
Refactor Mat_v5 to pass Matfiles around

Instead of passing the IO stream and swap_bytes boolean around separately, pass the matfile to the higher level functions instead. This will allow them to have access to the subsystem.
Add an outline for the read_opaque function
Use a concrete-typed IO in the Matfile

(Via parameterization).  This improves performance now that I pass around the matfile more
First pass at parsing the subsystem

Still need to add tests and implement parsing of nested objects