Rename binary/abstract booleans from 1 to true

For niftools#76

For reduced ambiguity.  Adapting for this in Python is as easy as

```py
# XML Booleans
XML_TRUE = ["true", "1"]
XML_FALSE = ["false", "0"]

# Example
self.is_abstract = get('abstract') in XML_TRUE
```

Note "True" and "False" are not valid booleans in XML.  Only lowercase.