Revisited membership/subscription

#11 #4 #1 
Correctly handles subscription length
No longer trusts user for prices
Only available to admins
do not install mykoop-database with npm. it is not a npm dependency it's a mykoop dependency. Basically if you write something like `require("mykoop-utils")` then you add mykoop-utils to npm dependencies. But if you do `this.getModuleManger().get("database);` Then it's a mykoop - dependency (see line 6 of this file)