Add "simple" TLRUCache example

[tkem]

The TLRUCache docs currently contain only an example for supplying a custom ttu function using datetime and a custom timer argument.
For clarity, an example using the default timer=time.monotonic and a ttu function returning a numeric value in seconds should be also added before that, to avoid confusion regarding the types involved in cache expiration. Maybe something like this should also be added to the TTLCache docs.
Also unit tests using alternative "time" types should be provided.