Use built-in C type names
Replace StrPack structs with immutables

This should improve performance, both for loading the module and for
reading data.  This passes all included tests and the tests for MAT.jl,
but I don't think test coverage is complete, so please test on your
data sets before merging.
Merge pull request #20 from simonster/remove-strpack

Use built-in C type names and replace StrPack usage with immutables