Add bundler gem tasks

```
$ rake -T
rake build    # Build double_entry-0.1.0.gem into the pkg directory
rake install  # Build and install double_entry-0.1.0.gem into system gems
rake release  # Create tag v0.1.0 and build and push double_entry-0.1.0.gem to Rubygems
rake spec     # Run RSpec code examples
```