[#438] first stab at this one

work in progress (not working yet)
[#438] further fixes and cleaning

Moved all specific code to the parser to simplify the converter as much
as possible
[#438] fixed attributes parsing for all nodes

the TMX file is now successfully converted !
still need more debug though, as the converted object is not then
properly loaded (probably other inconsistency between the native json
format and the converted one)
[#438] Fixed parsing of all converted layers

Still nothing display though, might be an issue with the display order
not being preserved
[#438] more fixes after some heavy debug session

still need further cleaning and some code rewrite, but almost there !
(time to go home in the meanwhile)
[#438] fixed an issue where `visible` is set to false if the original attribute is not set

Map is visible now ! remaining issue seems to be on the collision layer
[#438] fixed collision layer property parsing

TODO :
- code cleaning (a lot)
- fix a regression with “native” json loading
[#438] Remove the (now) un-necessary XML parsing code
[#438] cleaned up loader.js and moved the converter under me.TMXUtils
[#438] properly fixed collision layer property tiles parsing
[#438] fixed a regression with object parsing from a native json tmx map
Sync'ed the exported json file with the original XML mal
[#438] further cleaning and code refactoring

One bug left now (default z order is wrong)
[#438] Fixed default z ordering for converted XML file, and ImageLayer display
[#438] fixed parsing of the Tiled transparency attribute for Image

also refactored some code
[#438] fixed an exception in Firefox about the deprecated Attr `nodeName` and `nodeValue`

@see https://developer.mozilla.org/en-US/docs/Web/API/Attr
[#438] more simple and cleaner code

still have not figured out how to fix it property for Firefox ! to be
continued… tomorrow ;)
[#438] fixed xml parsing/conversion with Firefox

Also renamed `draw order` to `_draworder`
[#438] small clean-up (debug code and lint errors)
[#454] Added the Tiled `type` field to me.TMXObject