Add sqlserver input plugin

closes #589

plugins/inputs/sqlserver/README.md

@@ -0,0 +1,50 @@
+# SQL Server plugin
+
+This sqlserver plugin provides metrics for your SQL Server instance. 
+It currently works with SQL Server versions 2008+. 
+Recorded metrics are lightweight and use Dynamic Management Views supplied by SQL Server:
+```
+Performance counters  : 1000+ metrics from sys.dm_os_performance_counters
+Performance metrics   : special performance and ratio metrics
+Wait stats 			  : wait tasks categorized from sys.dm_os_wait_stats
+Memory clerk		  : memory breakdown from sys.dm_os_memory_clerks
+Database size         : databases size trend from sys.dm_io_virtual_file_stats
+Database IO			  : databases I/O from sys.dm_io_virtual_file_stats
+Database latency	  : databases latency from sys.dm_io_virtual_file_stats
+Database properties   : databases properties, state and recovery model, from sys.databases
+OS Volume             : available, used and total space from sys.dm_os_volume_stats
+CPU				      : cpu usage from sys.dm_os_ring_buffers
+```
+
+You must create a login on every instance you want to monitor, with following script:
+```SQL 
+USE master; 
+GO
+CREATE LOGIN [telegraf] WITH PASSWORD = N'mystrongpassword';
+GO
+GRANT VIEW SERVER STATE TO [telegraf]; 
+GO
+GRANT VIEW ANY DEFINITION TO [telegraf]; 
+GO
+```
+
+Overview
+![telegraf-sqlserver-0](https://cloud.githubusercontent.com/assets/16494280/12538189/ec1b70aa-c2d3-11e5-97ec-1a4f575e8a07.png)
+
+General Activity
+![telegraf-sqlserver-1](https://cloud.githubusercontent.com/assets/16494280/12591410/f098b602-c467-11e5-9acf-2edea077ed7e.png)
+
+Memory
+![telegraf-sqlserver-2](https://cloud.githubusercontent.com/assets/16494280/12591412/f2075688-c467-11e5-9d0f-d256e032cd0e.png)
+
+I/O
+![telegraf-sqlserver-3](https://cloud.githubusercontent.com/assets/16494280/12591417/f40ccb84-c467-11e5-89ff-498fb1bc3110.png)
+
+Disks
+![telegraf-sqlserver-4](https://cloud.githubusercontent.com/assets/16494280/12591420/f5de5f68-c467-11e5-90c8-9185444ac490.png)
+
+CPU
+![telegraf-sqlserver-5](https://cloud.githubusercontent.com/assets/16494280/12591446/11dfe7b8-c468-11e5-9681-6e33296e70e8.png)
+
+Full view
+![telegraf-sqlserver-full](https://cloud.githubusercontent.com/assets/16494280/12591426/fa2b17b4-c467-11e5-9c00-929f4c4aea57.png)