Remove grapher and exporter support

Remove grapher and exporter support #4

This makes spectralogic/rrd more flexible in that librrd/rrdtool
doesn't need to be compiled with rrdGraph or rrdXport support
(like on BP), which vail/goserver don't need.

Add DaemonInfo and UpdateNoDaemon functions

Daemon info calls the rrd info function with the daemon parameter.

UpdateNoDaemon calls rrd_updatex with the RRD_SKIP_PAST_UPDATES flag, which allows the user to write data previous to the last update.

Add DaemonFlush and fix memory leak

This adds DaemonFlush, which issues a flush command to the given daemon, for the given RRD file. Also, fix a memory leak in DaemonFetch where the daemon C string wasn't being freed.

Add ability to use rrdcached for updates

Add ability to use rrdcached for updates (#1)

* Plumb rrdc_update into the c code (and then the go code via rrdDaemonUpdate)

* Add a daemon *cstring member to the Updater struct

* if the Updater's deamon is not nil, use rrdDaemonUpdate instead of rrdUpdate

Add the ability to use the RRD daemon when fetching

Add DaemonFetch function, which takes the daemon address, and attempts to connect to and use the RRD daemon for fetch.

If the daemon connection fails, fall back to reading the files.