Support for different "bases" with multiples of deltas

[wesm]

Want a sane way to be able to do like

ts.resample('5min', base=2)

base=0 should be the default (as it is now)

[wesm]

After #1165, this should be pretty straightforward to needle in. Need to decide about API, though.

[wesm]

outcome:

In [6]: ts
Out[6]: 
2000-01-01 00:00:00    0.775840
2000-01-01 00:00:01    0.227596
2000-01-01 00:00:02   -1.457081
2000-01-01 00:00:03    0.734133
2000-01-01 00:00:04    0.356800
2000-01-01 00:00:05    0.981898
2000-01-01 00:00:06   -0.160146
2000-01-01 00:00:07   -1.960776
2000-01-01 00:00:08   -1.457221
2000-01-01 00:00:09   -0.829302
2000-01-01 00:00:10    0.413782
2000-01-01 00:00:11    1.550657
2000-01-01 00:00:12    0.324632
2000-01-01 00:00:13   -0.951606
2000-01-01 00:00:14    0.173150
...
2000-01-01 00:39:45   -0.989591
2000-01-01 00:39:46    0.659910
2000-01-01 00:39:47    0.961344
2000-01-01 00:39:48    0.030223
2000-01-01 00:39:49    1.165185
2000-01-01 00:39:50   -0.011536
2000-01-01 00:39:51    0.476151
2000-01-01 00:39:52    0.290827
2000-01-01 00:39:53    0.592913
2000-01-01 00:39:54   -0.597697
2000-01-01 00:39:55    0.622408
2000-01-01 00:39:56   -1.543871
2000-01-01 00:39:57    1.602200
2000-01-01 00:39:58   -0.382568
2000-01-01 00:39:59    0.327734
Freq: S, Length: 2400

In [7]: ts.resample('5min')
Out[7]: 
2000-01-01 00:00:00    0.775840
2000-01-01 00:05:00   -0.005698
2000-01-01 00:10:00    0.078389
2000-01-01 00:15:00   -0.018959
2000-01-01 00:20:00    0.013095
2000-01-01 00:25:00   -0.015351
2000-01-01 00:30:00    0.037673
2000-01-01 00:35:00    0.083128
2000-01-01 00:40:00    0.099568
Freq: 5T

In [8]: ts.resample('5min', base=3)
Out[8]: 
2000-01-01 00:03:00    0.070912
2000-01-01 00:08:00   -0.018604
2000-01-01 00:13:00    0.091425
2000-01-01 00:18:00   -0.073576
2000-01-01 00:23:00   -0.022931
2000-01-01 00:28:00    0.115416
2000-01-01 00:33:00   -0.025325
2000-01-01 00:38:00    0.098755
2000-01-01 00:43:00    0.166773
Freq: 5T