New features: custom timestamp file and cache managment

For my case I need a file with fake timestamp per process. I cannot use environment variable to change time offset, because of use python subprocess to run my application with faketime (no external changes to the enviroment are possible). Using single file would limit application testing to a single thread per user/machine. That's why I added custom timestamp file. 
Changes to these files might also be quite often, and that might be per test case option to turn caching off. So as not to keep two different libraries I decided to introduce another variable. Additional feature is possibility to set custom value for cache duration.