expose file creation properties to R (#94)

[ilia-kats]

No description provided.

[grimbough]

Thanks for the pull request, it's now been merged into the master branch.

I made a few modifications to H5Pset_shared_mesg_index() and H5Pget_shared_mesg_index(). For the former the mesg_type_flags should now provided as a string representing the type of message, rather than requiring the user to know the integer those constants map to. Possible options can be found with h5const(type = "H5O_SHMESG_FLAG").

This is reflected in the output from H5Pget_shared_mesg_index(), which is now a list where the first element is a character vector containing the message type and the second element is an integer vector with the minimum size.

I also added some unit tests to ensure the getters and setters were working.