Fixed shift.time issue #268

[pverspeelt]

added S3method(shift.time, xts) to the namespace.

Calling methods(shift.time) now returns [1] shift.time.xts*

Running the following code will now work for xts objects.

df1 <- data.frame(dates = Sys.time() + 1:10, 
                  values = 1:10)

df1_xts <- xts(df1$values, order.by = df1$dates)
shift.time(df1_xts, n = 60)

[joshuaulrich]

I reworded the commit message to align with the commit messages section of the contributing guide. The Git commit message is much more important to me than the PR message on GitHub. The commit message is part of the repository, and exists independently of the hosting platform (e.g. GitHub, GitLab, Bitbucket, etc), while the PR message is specific to GitHub.