ARROW-2789: [JS] Add iterator to DataFrame
ARROW-2789: [JS] Add iterator to DataFrame

Add an iterator to `FilteredDataFrame` and the `DataFrame` interface

Author: Brian Hulette <hulettbh@gmail.com>

Closes #2215 from TheNeuralBit/dataframe-improvements and squashes the following commits:

dba0e618 <Brian Hulette> rename length getter back to count()
a43269f5 <Brian Hulette> fix tests
664a053c <Brian Hulette> Add iterator and readonly length member

laurentgo/arrow,xhochy/arrow,xhochy/arrow,pcmoritz/arrow,wesm/arrow,itaiin/arrow,laurentgo/arrow,majetideepak/arrow,apache/arrow,cpcloud/arrow,tebeka/arrow,itaiin/arrow,renesugar/arrow,apache/arrow,icexelloss/arrow,xhochy/arrow,kou/arrow,xhochy/arrow,itaiin/arrow,pcmoritz/arrow,renesugar/arrow,apache/arrow,tebeka/arrow,cpcloud/arrow,itaiin/arrow,icexelloss/arrow,icexelloss/arrow,cpcloud/arrow,itaiin/arrow,kou/arrow,icexelloss/arrow,pcmoritz/arrow,cpcloud/arrow,renesugar/arrow,wesm/arrow,itaiin/arrow,laurentgo/arrow,laurentgo/arrow,wesm/arrow,itaiin/arrow,kou/arrow,apache/arrow,kou/arrow,renesugar/arrow,renesugar/arrow,pcmoritz/arrow,icexelloss/arrow,renesugar/arrow,xhochy/arrow,majetideepak/arrow,wesm/arrow,cpcloud/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,tebeka/arrow,renesugar/arrow,wesm/arrow,xhochy/arrow,renesugar/arrow,itaiin/arrow,tebeka/arrow,icexelloss/arrow,pcmoritz/arrow,majetideepak/arrow,majetideepak/arrow,wesm/arrow,cpcloud/arrow,majetideepak/arrow,icexelloss/arrow,xhochy/arrow,kou/arrow,xhochy/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,pcmoritz/arrow,wesm/arrow,xhochy/arrow,xhochy/arrow,cpcloud/arrow,majetideepak/arrow,pcmoritz/arrow,wesm/arrow,itaiin/arrow,tebeka/arrow,renesugar/arrow,cpcloud/arrow,majetideepak/arrow,apache/arrow,renesugar/arrow,laurentgo/arrow,pcmoritz/arrow,cpcloud/arrow,kou/arrow,icexelloss/arrow,apache/arrow,apache/arrow,xhochy/arrow,kou/arrow,itaiin/arrow,laurentgo/arrow,renesugar/arrow,kou/arrow,laurentgo/arrow,apache/arrow,majetideepak/arrow,wesm/arrow,itaiin/arrow,pcmoritz/arrow,laurentgo/arrow,itaiin/arrow,pcmoritz/arrow,xhochy/arrow,apache/arrow,renesugar/arrow,majetideepak/arrow,renesugar/arrow,tebeka/arrow,kou/arrow,kou/arrow,icexelloss/arrow,pcmoritz/arrow,itaiin/arrow,apache/arrow,pcmoritz/arrow,laurentgo/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,kou/arrow,tebeka/arrow,wesm/arrow,xhochy/arrow,xhochy/arrow,pcmoritz/arrow,majetideepak/arrow,tebeka/arrow,icexelloss/arrow,pcmoritz/arrow,itaiin/arrow,majetideepak/arrow,tebeka/arrow,apache/arrow,majetideepak/arrow,kou/arrow,apache/arrow,renesugar/arrow,tebeka/arrow,laurentgo/arrow,itaiin/arrow,majetideepak/arrow,pcmoritz/arrow,cpcloud/arrow,laurentgo/arrow,majetideepak/arrow,icexelloss/arrow,xhochy/arrow,wesm/arrow,renesugar/arrow,laurentgo/arrow,kou/arrow,cpcloud/arrow,cpcloud/arrow,majetideepak/arrow,kou/arrow,laurentgo/arrow