Add logic to get and set functions for default value

## Description

The changes in this pull request add logic to the <code> rb_ct_field_get_default</code> and <code> rb_ct_field_set_default </code> functions.  I added assertions to <code>test_ct_field</code> to ensure the functions are working properly.  As of this writing, those tests are passing.

![image](https://f.cloud.github.com/assets/1584123/1535337/7cb4d692-4ca2-11e3-9431-c6669e80d204.png)