Split NfsCollector in Nfs3Collector and Nfs4Collector (#28)

* Split NfsCollector in Nfs3Collector and Nfs4Collector

* Add documentation

collectors/nfs3Metric.md

@@ -0,0 +1,39 @@
+
+## `nfs3stat` collector
+
+```json
+  "nfs3stat": {
+    "nfsstat" : "/path/to/nfsstat",
+    "exclude_metrics": [
+      "nfs3_total"
+    ]
+  }
+```
+
+The `nfs3stat` collector reads data from `nfsstat` command and outputs a handful **node** metrics. If a metric is not required, it can be excluded from forwarding it to the sink. There is currently no possibility to get the metrics per mount point.
+
+
+Metrics:
+* `nfs3_total` 
+* `nfs3_null` 
+* `nfs3_getattr` 
+* `nfs3_setattr` 
+* `nfs3_lookup` 
+* `nfs3_access` 
+* `nfs3_readlink` 
+* `nfs3_read` 
+* `nfs3_write` 
+* `nfs3_create` 
+* `nfs3_mkdir` 
+* `nfs3_symlink` 
+* `nfs3_remove` 
+* `nfs3_rmdir` 
+* `nfs3_rename` 
+* `nfs3_link` 
+* `nfs3_readdir` 
+* `nfs3_readdirplus` 
+* `nfs3_fsstat` 
+* `nfs3_fsinfo` 
+* `nfs3_pathconf` 
+* `nfs3_commit` 
+