Add ability to run configurable "preupdate" and "postupdate" shell scripts during image file writing.

Preupdate runs before replacing the .png, postupdate runs after. Both wait for completion before continuing, so scripts should be fast, or fork into the background themselves if appropriate. The image file name is provided as an argument.

Expected use-cases:
-Run optipng or similar in preupdate
-Upload to webserver or flush caches in postupdate

I'm sure there are others too. :)
Add upgrade version check
Bump to 1.5
Update renderdata/railcraft6-texture.txt

Added "Sandy Brick" to railcraft6-texture.txt
Update renderdata/railcraft6-texture.txt

changed to right file...
Merge pull request #13 from Blast0r/master

Update railcraft6-texture.txt to support "Sandy Brick"
Added "blockIronScaffold" texture to IC2
Update renderdata/ic2-texture.txt

Added "blockIronScaffold"
Make sure updated JSON files are fresh (no extra)
Merge pull request #15 from Blast0r/master

Added "Iron Scaffold" to ic2-textures and renderdata
Merge pull request #11 from Maeyanie/master

Add ability to run configurable shell scripts before/after writing tile images
Remove Java 7 specific code in Maeyanie's PR
Switch to more structured configuration settings for image update exits
Prevent overlapping file updates on JsonFile update
Start Pam's Mods Support
Add Pam's HC Candle mod
Add Pam's HC Cherry mod
Add Pam's HC Chilipepper Mod
Add Pam's HC Corn Mod
Add Pam's HC Cotton mod