Implement DatetimeIndex

[sethmlarson]

See pandas.DatetimeIndex and date_range. End goal is to be able to visualize time-series data on a histogram.

[stevedodson]

There are a couple of parts to this issue (which could be split into parts):

1. Plot a time series based on x=datetime column (potentially downsampling in ES)
2. Bucket data into hourly/daily etc. buckets
3. Set index to DateTimeIndex and use date_range and other methods

e.g.