Add installed check for fping

[baweaver]

Current errors just raise no such file fping

[penguinpowernz]

Hmmm, sorry dude I'm not sure that satisfies the principle of least surprise. People are expecting to get an IP address from there, not an error message in a string.

I like the general idea though. Perhaps we could instead raise an error like Net::Fping::BinaryNotFound or Net::Fping::NotInstalled using your fping_installed? method as the check?

def BinaryNotFound < StandardError; end

def fping_installed!
  raise BinaryNotFound unless fping_installed?
end

But we should change the check to use %x[which fping] instead of system "which fping" as the latter could cause the library to print out the fping binary location everywhere it is called. Using the %x[] stanza will assign the output of which to the ether.