server: unexport loadRules method
server: refactor loadRules, so it return the slice of rules
server: add NewServer function

This function is used as a construtor for the Server type, building the
frontend server from a rule file.
server: make loadRules a function instead of a method

It does not interact with the server object, so it does not make any
sense to be a method.
tests: remove test that does not make any sense anymore
server: added more tests to NewServer function