add variable global time via time zone

[lorenzwalthert]

Need to be able to work with global time because otherwise commit order might be parsed wrongly.
I.e. below, 9bc41062154411e66a88662d61293559c218c03c came before 01dc289b84e196d5234dc28652f4a319025acf7a although the date shown inclines the reader think the opposite
https://github.com/tidyverse/dplyr/commits/master/tests/testthat/test-colwise.R
https://github.com/tidyverse/dplyr/commits/master/tests/testthat/test-colwise-mutate.R

[lorenzwalthert]

This should probably be fixed before #15 is merged.

[lorenzwalthert]

Ok, does not solve the problem.