[bugfix] Only compare matching length vectors

[mschilli87]

Ensure length(special.days) == 1L before comparing to "weekends". This is requried for newer versions of R.

As far as I can tell, this does not limit any functionality, but masks the informative error message when specifying the wrong number of days (i.e. 365 for a leap year) with an uninformative one.

This commit fixes issue #30.

@R-CoderDotCom: I submitted calendRio v.02.1 containing this fix to CRAN and thought I offer it upstream as well.