added calc.zcent() for computing the standardized values of a variable for each subject
added calc.fun() for computing arbitrary summary statistics for each subject
in lagvar(), added time argument (that allows for computing the time difference between lagged observations) and made the examples for this function more complete
in lagvar(), moved id argument to the second position and made obs argument optional
added aggreg() function
added nsub(), calc.mean(), and calc.mcent() functions
added calc.nomiss() for computing the number of non-missing values for a particular variable
added check.nodup() to check if a variable has no duplicated values for each subject
added check.nomiss() and updated check.timeinvar() to provide more output options
started using Travis CI (https://travis-ci.org/wviechtb/esmpack)
started on pkgdown website
added check.timeinvar() and get.timeinvar() functions
added combitems() and lagvar() functions
initial setup for GitHub