Made the CuraEngine repo configurable

  * Added a check for the environment variable CURA_ENGINE_REPO, if it's not
    set use https://github.com/Ultimaker/CuraEngine otherwise try to use
    whatever it's set to
Add imageToMesh tool.
Update changelog.
Merge pull request #590 from grimmy/SteamEngine

Made the CuraEngine repo configurable
Update commandline programmer.
Merge branch 'SteamEngine' of github.com:daid/Cura into SteamEngine
Merge remote-tracking branch 'upstream/SteamEngine' into SteamEngine
add new zh-CN