Diff only selected components in .int_to_period

Subsetting to just the first six POSIX components in the creation of `to.per` (and doing so explicitly by name) eliminates the factor-related problems caused by the addition the "zone" component in `R 3.1.0`, as described in hadley/lubridate#243.

Closes #243.
Merge pull request #244 from mmparker/patch-1

Diff only selected components in .int_to_period