add option to poll stats for all nodes in the cluster

[dan-cleinmark]

Tested on a 6 node cluster (1 master, 5 data nodes). Might cause timeouts on very large clusters.

$ time curl -s "http://localhost:9200/_nodes/_local/stats" &> /dev/null
real    0m0.011s
user    0m0.000s
sys 0m0.004s

$ time curl -s "http://localhost:9200/_nodes/stats" &> /dev/null
real    0m2.164s
user    0m0.008s
sys 0m0.004s

[ewr]

@dan-cleinmark: So sorry about not seeing these before. Apparently I wasn't watching my repo, and therefore didn't get notified. My fault.

We're having a bigger cleanup discussion in #4, and I want to make sure that your submissions get pulled in as part of that effort.