Use Simplecov, Gem::PackageTask, and RSpec in Rakefile

This is a separate branch from my other PR for UDP support. spicycode-rcov seems to  be unmaintained (<tt>gem</tt> doesn't even know what it is any more) and [Rcov](https://github.com/relevance/rcov) doesn't work in Ruby 1.9, so I've migrated to SimpleCov.

Jeweler support has been replaced with by using a new :gemspec task
to generate <tt>simple-graphite.gemspec</tt> and by using Gem::PackageTask for
building new gem files.

The <tt>:test</tt> functions have been removed and replaced with
Rspec tasks.

A test for hostname validation has been added. This brings coverage
up to 100% on the v1.1.1 codebase.