The ROXML library handles boolean XML true false fields

```
I thought I would put this PR out there for consideration, discussion, and for a possible merge.
However, please note that this PR would break certain coding patterns around boolean XML nodes. 
These breakages are explained below.
```


http://roxml.rubyforge.org/rdoc/classes/ROXML/ClassMethods/Declarations.html
However, in the code base there is a lot of duplicate predicate methods
that could be eliminated by simply adding a '?' to the end of
xml_accessor e.g.
by setting:
`xml_accessor :active?, :from => 'Active'`
this would eliminate:
```
def active?
  active.to_s == 'true'
end
```

This PR would possibly break code that relies on an unset attribute to
be false e.g.
Currently, if active is not set it will return false if the predicate
method from above has been created.
After the PR it will return nil.
Moreover, after this PR there is no longer a getter method e.g. `model.active`.
Currently, a call to `model.active` would return the string "false" or "true". After the PR
it will be an undefined method.