Microtime rewrite

as stated in issue https://github.com/alexfernandez/loadtest/issues/5 it may be a good idea to not use the native module microtime as there'a built in solution for high resolution timers. 
the microtime modul has to be compiled and therefore demands for a working compilation environment. 

this PR utilizes this hrtimer and removes microtime.

hope you like it!

the outher small changes are just due to my ocd kicking in when my editor highlights a missing semicolon... ;)