Add support for "--daemon" option

[robertgrimm]

RRD::Wrapper.fetch and RRD::Wrapper.last_update, among others possibly, do not support the "rrdtool fetch"'s and "rrdtool lastupdate"'s --daemon argument (so a flush command can be sent to the daemon prior to reading the RRD files). I see 2 options:

1. Use rrd_fetch/lastupdate, instead of rrd_fetch_r/rrd_lastupdate_r and let those handle the --daemon option.

2. Continue using the rrd_fetch_r/rrd_lastupdate_r functions and handle the --daemon options in Ruby by calling rrdc_flush if the option is present.

RRD::Wrapper.info does support it since it passes all options directly to rrd_info.