Add console task for development funsies
Add protobuf:compile rake task for external use

The `protobuf:compile` task can be utilized in any project or gem that
uses protobuf. Open up the Rakefile of the including project and add:

```ruby
require ‘protobuf/tasks’
```

The `compile` task accepts three arguments: package, source, and
destination. Package does not have a default value and must be
specified. Source’s default is ‘defintitions’. Destination’s default is
`lib`. Plugin’s default is `ruby`, and is overridable in case you want
to utilize the descriptors to make your own plugin generator.

The composed command to protoc is printed to the terminal and then the
process is exec’d with that command.
Add protobuf:clean task for external use

To get this task simply `require ‘protobuf/tasks’` in your Rakefile.

Clean will ask for confirmation of removing previously compiled pb.rb
source files. You can skip the confirmation step by providing
PB_FORCE_CLEAN=1. A force clean is automatically invoked from the
`compile` task unless PB_NO_CLEAN=1 is specified.

The package argument must be specified. The destination argument
defaults to `lib`.
Merge pull request #142 from localshred/bj/rake-tasks

Add protobuf:clean and protobuf:clean rake tasks for external use