Use a primitive field mapping from symbol to field class

Allows us to generate “prettier” message code that use symbols for all
the primitive types in the definition.

```ruby
# old
class Foo
  optional ::Protobuf::Field::StringField, :bar, 1
end

# new
class Bar
  optional :string, :foo, 1
end
```
Change the generator to output the symbol type name for primitive field definitions
Recompile all protos in the gem with the latest generator changes
Cleanup rake task descriptions
Merge pull request #170 from localshred/bj/symbol-fields

Use a primitive field mapping from symbol to field class