Fix default value in WJEDouble get.
Merge pull request #19 from JoseGoncalves/master

Fix default value in WJEDouble get.
Added callbacks that can be used when writing a document to allow filtering or changing "pretty" values
- selector fix: use WJEChild instead of WJEGet where appropriate
- added some documentation about selectors/WJEGet/WJEChild
fix compiler warnings
 still requires "-Wno-char-subscripts" when compiled with "-Wall"
Style changes