NullCalculationWarnings make csvstat output hard to read

Created by: onyxfish

csvstat examples/realdata/FY09_EDU_Recipients_by_State.csv

...
/Users/cgroskopf/src/agate/agate/aggregations/mean.py:34: NullCalculationWarning: Column "Code" contains nulls. These will be excluded from Mean calculation.
/Users/cgroskopf/src/agate/agate/aggregations/median.py:37: NullCalculationWarning: Column "Code" contains nulls. These will be excluded from Median calculation.
/Users/cgroskopf/src/agate/agate/aggregations/stdev.py:36: NullCalculationWarning: Column "Code" contains nulls. These will be excluded from StDev calculation.
  3. Code
	Number
	Nulls: True
	Min: 1
	Max: 72
	Sum: 1549
	Mean: 29.78846153846153846153846154
	Median: 29.5
	Standard Deviation: 16.77455652487823123699374366
	Unique values: 53
	5 most frequent values:
		1:	1
		2:	1
		4:	1
		5:	1
		6:	1

Haven't decided if this is a 1.0 issue or not. Tentatively in.