Answers are now typed objects

All answers are parsed into one of the these types:
```
Smartdown::Model::Answer::Date
Smartdown::Model::Answer::MultipleChoice
Smartdown::Model::Answer::Salary
```
The Question models are responsible for specifying the type of answer expected.

This is the first step in being able to validate answers.
Is there no way to avoid the extra `to_s`'s? It seems like it would be really easy to forgot/miss in the future.