add `test` target in Makefile

mocha as devel dependency
Options for jshint
Implement dates restriction

Calendar has new `min` and `max` functions to define inclusive range of
valid dates.

Range checking is performed by DayRange class.
When rendering calendar dates ourside of the range are rendered with
`invalid` CSS class, so that can they be styled differently from valid
dates.

Calendar ignores attempts to select invalid date, although is allows to
show it.