Made illegal key error message a bit clearer
Added `size()` method to the config class
Added `removeSection()` to the config class
Minor fixes in comments/docs
Fixed problem with temp file not always getting created in the right folder and sometimes not getting cleaned up properly after error
Improved tests for changing and writing configurations
Implemented `set`, `remove` and `remove-section` and added `--system`, `--user` and `--local` flags