Merge pull request #7 from brtz/master

Merge master back into develop;
[CHANGE] implement RAM-caching and moved caching to SrvinvCache class
[CHANGE] do not use json internally
[FIX] do not fail on empty srvinv replies
[CHANGE] rewrote caching class
Merge pull request #8 from brtz/develop

optionally cache data in RAM instead of json files