Suggest capabilty removes unneeded graphs

Added checks to update_data to not graph things that:
a) Aren't supported in the mysql version
b) Aren't configured (e.g. bin logs and replication)
Add a "plugin writing tips" page

This will complement the basic "how to write a plugin" page, which should be as basic as
possible (but no more than that).
Mark example as a code block
Add galera monitoring variables

Based on work by <mrten+munin@ii.nl> submitted here:
https://github.com/kjellm/munin-mysql/commit/19aa49f14368a35909cf4f23b4fc591cad5e3b67
better handling of undefined data
fix wsrep_flow_control_sent/wsrep_flow_control_recv as DERIVES and make the wsrep_flow_control_recv -ve on same graph
galera flow control paused to own graph with warning at 0.1 and critical at 0.9
syntax fixes
pause events received not graphed. try adding label.
quote min/max values on wsrep_flow_control_paused, added wsrep_local_{recv|send}_queue_{min|max}, do +- graph on wsrep_local_{recv|send}_queue
add wsrep_cert_index_size to  wsrep_distance graph
add bits to wsrep_concurrency - requires https://github.com/codership/galera/pull/57
wsrep_writesets graph to +-
wsrep_writesetbytes graph additions for key/data/other bytes and into an area graph
remove wsrep_avgwritesetbytes
galera commit - title fix
add wsrep_flow_paused_ns and carify the titles/labels on wsrep_flow_paused
wsrep_flow_paused_ns not a GAUGE
remove poor quality wsrep_avgreplicated/wsrep_avgreceived data calculation
Remove apply and commit count from p/mysql

These haven't been accepted upstream yet
(https://github.com/codership/galera/pull/57)