Portable Arduino

Use Arduino on USB flash stick and without affecting anything on your PC.

To use this feature, simply make a folder named "portable" in the application
root folder where there are "hardware", "examples", "lib" and others.
In this mode, file association setting is disabled in the preferences dialog.

The sketch book folder is inside the "portable" folder. This may be confusing
especially for Mac OSX "Arduino.app" folder.
fixed a bug introduced by my rebase after 24bef6b.
see #1211: formatted code, moved RelativePath code into FileUtils
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