allow specifying executable to run including other gems executables

allow for:

- specify custom executable
- executable from other gems to be used
- default (prefixed) parameters for the executable
- improved content tests (use `.should =~ ...` instead of `.grep(...).should_not be_empty`)
 
example `config/warble.rb` (requires a `Rakefile`):
```ruby
Warbler::Config.new do |config|
  config.gems = [ "rake" ]
  config.executable = ["rake", "bin/rake"]
  config.executable_params = "do:something"
end
```