BUG: allow Timestamp comparisons on the left

Now tested with Series and DataFrame
BUG: Fix Timestamp compares to NaT

For example

    Timestamp('now') > pd.NaT

should return the same result as

    pd.NaT < Timestamp('now')

but before this commit it didn't because when Timestamp was on the left, the
values were compared whereas if NaT is on the left then the truth values are
hard-coded based on rules like NaN.
PERF: add vbench for DatetimeIndex unions/intersetions
PERF: add Timestamp <-> Series comparison vbench
Merge pull request #4983 from cpcloud/series-timestamp-compare

BUG: allow Timestamp comparisons on the left