Add number of users to 'system' plugin

see #235

plugins/inputs/system/SYSTEM_README.md

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+# System Input Plugin
+
+The system plugin gathers general stats on system load, uptime,
+and number of users logged in. It is basically equivalent
+to the unix `uptime` command.
+
+### Configuration:
+
+```toml
+# Read metrics about system load & uptime
+[[inputs.system]]
+  # no configuration
+```
+
+### Measurements & Fields:
+
+- system
+    - load1 (float)
+    - load15 (float)
+    - load5 (float)
+    - n_users (integer)
+    - uptime (integer, seconds)
+    - uptime_format (string)
+
+### Tags:
+
+None
+
+### Example Output:
+
+```
+$ telegraf -config ~/ws/telegraf.conf -input-filter system -test
+* Plugin: system, Collection 1
+> system load1=2.05,load15=2.38,load5=2.03,n_users=4i,uptime=239043i,uptime_format="2 days, 18:24" 1457546165399253452
+```