Add RakeTask

This allows you to easily use `fpm` in a Rakefile without a bunch of backticks or other non-Ruby ugliness. For example, given a Rakefile:

```ruby
require "fpm/rake_task"

FPM::RakeTask.new(:awesome_pkg, source: :dir, target: :tar) do |pkg|
  pkg.args = %w(bin/)
end
```

Run `rake awesome_pkg`:

```
Created package {:path=>"awesome_pkg.tar"}
```