Getting MIN, MAX and LAST

[msiebuhr]

Currently, the library only supports getting AVERAGE. It should be able to get MIN, MAX and LAST as well.

I imagine the current fetch(from, to, callback) could be changed to fetch(type, from, to, callback) (and possibly shortcuts like fetchAvg(from, to, callback) if you're into that sort of thing).

[msiebuhr]

Just discovered HWPREDICT, MHWPREDICT, SEASONAL, DEVSEASONAL, DEVPREDICT and FAILURES per http://oss.oetiker.ch/rrdtool/doc/rrdcreate.en.html