implement floor and ceiling; closes #64

[lsilvest]

For durations we match the xts::align.time function, and for periods we pay a bit the need to check for time zone changes, but it's a very acceptable price. Furthermore, one would use periods only for granularities of one hour or more, so in practice it should never be an issue.

                 test replications elapsed relative
3 nanotime (duration)           10   5.082    1.000
2   nanotime (period)           10  11.586    2.280
1                 xts           10   5.391    1.061

[eddelbuettel]

Looks fabulous. Thanks for all the documentation and tests!