Apply MIT license to md2k7 art

[21:48:14] <md2k7> feel free to make it MIT if you like

contrib/debian/copyright

@@ -55,8 +55,7 @@ Comment: Icon Pack: Human-O2
 
 Files: src/qt/res/icons/transaction*.png
 Copyright: md2k7
-License: You are free to do with these icons as you wish, including selling,
+License: Expat
- copying, modifying etc.
 Comment: Site: https://bitcointalk.org/index.php?topic=15276.0
 
 Files: src/qt/res/icons/configure.png, src/qt/res/icons/quit.png,

doc/assets-attribution.txt

@@ -29,6 +29,7 @@ Designer: md2k7
 Site: https://bitcointalk.org/index.php?topic=15276.0
 License: You are free to do with these icons as you wish, including selling,
  copying, modifying etc.
+License: MIT
 
 Icon: src/qt/res/icons/configure.png, src/qt/res/icons/quit.png,
       src/qt/res/icons/editcopy.png, src/qt/res/icons/editpaste.png,