Allows a user to define a :prebuid rake task

This gets invoked before the build process.

Inspired by my difficulty in running commands before the build process, i wrote a blog post about it... I wanted to make it easier.

http://blog.markrickert.me/prepending-the-default-rubymotion-build-task

So now in the rakefile you can define a `:prebuild` task and it will run before the build process.

```ruby
desc "Prebuild Process"
task :prebuild do
  # Do something awesome and automated!
end
```

Thoughts?