Add a versioned Marshaller API to aid plugin development

[frejanordsiek]

Writing plugins (see Issue #65 ) is complicated by how features can be added, changed, and removed over time in this package. While it is straightforward to figure out the oldest version of hdf5storage a plugin might support, it is impossible to figure out how far into the future. This problem can be ameliorated to some degree by having a separately versioned Marshaller API that plugins can use and declare they are compatible with. Then, as long as the plugin uses no other features of hdf5storage, it should work with future versions for as long as the particular Marshaller API is supported. A given hdf5storage package version may support more than one Marshaller API version.

[frejanordsiek]

Added initial support to master branch in commit 817da0b . What Marshaller API version '1.0' is comprised of exactly still needs to be defined and documented.