Make lastMonth or nextMonth days pickable

It's frustrated when the lastMonth or nextMonth days can't be picked,
So I removed the limit.
Add BatchMode feature

You can configure to pick a whole week or month.
BatchMode feature readme
update moment.js to 2.6.0

need the moment().startOf('isoweek') in version 2.2
so update to most higher version.
Fix startDate and endDate limit when batchMode on
Merge pull request #26 from lingceng/master

Add Batch Mode to select a whole week or month
Make grey dates selectable