Fully support string,int,bool,float data types.

[nathanielc]

Kapacitor supports int and float operations currently. Add support for string and bool operations as well. This will enable full interaction with InfluxDB data sets since these are the same native types. This also means that the eval expressions should support returning and manipulating these types as well. Recent work #491 has enabled extending the expressions in this way.

Related issue and discussions:
#495

https://groups.google.com/forum/?pli=1#!topic/influxdb/TY_2IuuIxs8
https://groups.google.com/forum/?pli=1#!topic/influxdb/5ptiuF54DJI