lrucache support MutableMapping interface

[spumer]

now new methods available: pop, popitem, update (replaced), setdefault

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This PR makes lrucache object 100% compatible with dicts.
Actually i need one method - pop, but as i saw in commits history someone needs update method.
dict-like update method already provided by MutableMapping based on __setitem__ implementation.

I don't edit WriteThroughCacheManager and WriteBackCacheManager cause len for WriteBackCacheManager should be calculated too difficult ( len(cache) + len(store)? )

[jlhutch]

I like the idea of conforming to the abstract base class MutableMapping. The problem is, lrucache doesn't actually conform. PEP 3119 states that Mappings should have keys(), values(), etc. return something that conforms to Set. But, lrucache returns iterators, which don't. Side note, the decision to return iterators was a design mistake I made in the early days of pylru.

If there is specific functionality from the dict interface, that you want in lrucache, I think the easiest thing would be to implement it directly, at least for now. I just pushed some changes to master that may help you.

If you want lrucache to conform to MutableMapping, that requires changing the API and I don't want to break code for current users that expect to get an iterator. We could change the API in a new 2.0 release and list both on PyPI. I'm not sure if there is enough demand to justify that or not.