Remove needless Edge\UndirectedId

[clue]

Remove experimental leftover

• Always use Edge\Undirected instead of Edge\UndirectedId and remove Edge\UndirectedId
• 100% code coverage
• Documentation / changelog

Refs #27.

[clue]

Afaikt, this does not affect BC and does not need an addition to the changelog? The documentation for Vertex::createEdgeTo() has always referred to EdgeUndirected anyway.

Remove [WIP] marker, ready for review. I'll leave this open for a day or two, but this should be safe to be merged as-is.

[clue]

Just to add some context: The Edge\UndirectedId was the result of an experiment creating a graph with 100.000 vertices and 200.000 edges. PHP (version unclear, 5.3.?) used to segfault when we tested this around a year ago. It's been worked around by reducing the number of circular references, but imo this was never more than an experiment anyway. As such, we don't have any bug reports, detailed information or unit tests to reproduce this issue.

And in my opinion, if you're trying to work with Graphs that big, you're probably better off with using a full-fledged graph database instead of keeping the whole graph in memory (independent of the underlying graph library).

[clue]

Rebased on current master, fixed a now relevant reference in Algorithm\Tree\Undirected and added to README just in case.