Exact Weeks and Months beetween dates

Example:
 2014-06-18 & 2014-05-27 have 2 elapsed weeks (1 completed and start
 another), but only 1 complete week.
containsCalendar
containsDateBuilder
containsDate
Merge pull request #4 from caarlos0/feature/tests

Tests determinism
Merge pull request #3 from caarlos0/feature/exact-intervals

Exact Weeks and Months beetween dates