Correctly interpret logical matlab arrays as booleans when mat_dtype is set
DOC: io: update code comment and whosmat docstring
Merge pull request #504 from batterseapower/master

ENH: io: Present loaded logical Matlab arrays with a boolean dtype

There was already some code which attempted to do this in the Matlab
file reader, but it was broken. To be specific, the old code assigned to
a dtype to a local variable bool_dtype rather than self.bool_dtype, and
so later attempted uses of that dtype would be null/None and so would
not have an effect.

This change makes also the writer set the logical flag if you write out
an array of bools.