Added option create Moment from Moment.
Added method asArray.
Added method startOf.
Changed default value of unit for method diff.
Added method isBefore.
Fixed an error in the calculation of the difference between dates.
Changed method isBefore.

Beginning dates are set to the unit and the difference is calculated in seconds. It is therefore unnecessary refer unit to the method for calculate the difference and get back float number.
Added methods isAfter and isSame.
Changed README.