Added to day of week modifiers. startOfWeek() & endOfWeek()
Adding tests for Day Of Week Modifiers
Updating week date modifier methods to use ISO-8601.  Monday is 1st day of week.  Majority standard
Merge branch 'master' of https://github.com/travisjryan/Carbon into travisjryan-master
Moved what I think are modifier methods to the right section.
Updated comment to be more specific about which day is the start and end according to ISO-8601.
Being a little more specific to ensure the time is also updated.  Unnecessary unless you ran this test at precisely 00:00:00 which is possible.