More state cleanup in BaseField, alphabetizing

Finishing off what @tamird started in #205.

Alphabetize all methods in BaseField.

Rather than deleting values out of the options hash, reference the hash
from predicate functions.

Rename `getter_method_name` and `setter_method_name` to simpler
`getter` and `setter`.

Remove the excess options warning since we are generating all field
definitions.
alternatively, you could even make this class a Struct, which would give you the accessors and most of the initializer for free:
```ruby
BaseField = Struct.new(:message_class, :rule, :type_class, :name, :tag, :options) do

  def initialize(*)
    super
    validate_packed_field if packed?
    define_accessor
  end
end
```
that may be too terse for your taste, though.