Cross platform support for the 'processes' plugin

closes #798

plugins/inputs/system/PROCESSES_README.md

@@ -0,0 +1,58 @@
+# Processes Input Plugin
+
+This plugin gathers info about the total number of processes and groups
+them by status (zombie, sleeping, running, etc.)
+
+On linux this plugin requires access to procfs (/proc), on other OSes
+it requires access to execute `ps`.
+
+### Configuration:
+
+```toml
+# Get the number of processes and group them by status
+[[inputs.processes]]
+  # no configuration
+```
+
+### Measurements & Fields:
+
+- processes
+    - blocked (aka disk sleep or uninterruptible sleep)
+    - running
+    - sleeping
+    - stopped
+    - total
+    - zombie
+    - wait (freebsd only)
+    - idle (bsd only)
+    - paging (linux only)
+    - total_threads (linux only)
+
+### Process State Mappings
+
+Different OSes use slightly different State codes for their processes, these
+state codes are documented in `man ps`, and I will give a mapping of what major
+OS state codes correspond to in telegraf metrics:
+
+```
+Linux  FreeBSD  Darwin  meaning
+  R       R       R     running
+  S       S       S     sleeping
+  Z       Z       Z     zombie
+  T       T       T     stopped
+ none     I       I     idle (sleeping for longer than about 20 seconds)
+  D      D,L      U     blocked (waiting in uninterruptible sleep, or locked)
+  W       W      none   paging (linux kernel < 2.6 only), wait (freebsd)
+```
+
+### Tags:
+
+None
+
+### Example Output:
+
+```
+$ telegraf -config ~/ws/telegraf.conf -input-filter processes -test
+* Plugin: processes, Collection 1
+> processes blocked=8i,running=1i,sleeping=265i,stopped=0i,total=274i,zombie=0i,paging=0i,total_threads=687i 1457478636980905042
+```