Enable Cache to handle stale values

[hkraal]

As suggested by @bargulg in #22 there should be room for caches like Redis to clean up their own stale values. Currently the expired logic takes place in APIConnection.get() which decides if a cache would be valid or not.

My thoughts:

• Determining if the cache is expired is not up to APIConnection but the loaded caching plugin
• The caching plugin should decide if it wants to use a relative or absolute time
• The code in APIConnection.get() shouldn't be doing more then res = cache.get('key'), if res.valid: ... and cache.put('key')

[jonobrien]

I like this idea, we just need more fleshed out workflows for implementing those cache mechanisms, and how each should work, unless you want to leave that up to the end-users of PyCrest.

[jonobrien]

you could just set a flag for variable or absolute time as well, if they all could be used/tested in the same manner, reducing user confusion.