Update graph io to be able to process metadata

[pszufe]

With #12 a metadata functionality has been provided. Now we need to decide how to store such data. Perhaps a good option would be to use the BSON.jl library that has good stability as well as good portability across platforms.

The simple text representation that is currently developed in #11 also could be used in parallel - but here we need somehow to represent the metadata. A reasonable option would be to allow text representation if V and E in Hypergraph defintion are subtypes of a Real. Otherwise plain text save can be simply not supported for a Hypegraph.