add iterators for groups, attributes

[Vindaar]

Have some file aware iterators for groups and attributes.

While groups are stored in a table, the current way to iterate over the keys of the table sort of works, but is not very nice. This requires the user to call the visitFile proc before hand, which should of course not be necessary.

Iterator should thus check whether file has been visited already (add visited flag to H5FileObj maybe?), and if not perform visit to file before yielding group names etc.

[Vindaar]

Commits 3a49372 and 4e8c83a add basic iterators for groups and datasets within a file.

Iterator for attributes still missing, plus dataset iteration is still limited to all datasets within a subgroup (without support for recursive iterations over subgroups of the group being iterated over).

[Vindaar]

#32 also adds JSON based iterators called attrsJson. The name was chosen to not confuse users about what happens.