Fix: Added .gitignores to check-in Editor dirs

• Added empty `Assets/TouchScript/Editor` where needed using the .gitignore-placeholder trick (see http://stackoverflow.com/questions/115983/add-empty-directory-to-git-repository).
	» Running `./Build/compile.sh` was failing in multiple spots due to a `cp: directory …/TouchScript/UnityPackages/TouchScript.…/Assets/TouchScript/Editor does not exist` error.
	» Same failure when opening TouchScript.sln in Unity's MonoDevelop and trying to `Build TouchScript`.
	» Now builds fine on OS X with Unity 4.6 with a clean `git clone …/TouchScript.git && cd TouchScript && git submodule update --init --recursive && ln -s /Applications/Unity/Unity.app/Contents/Frameworks/Managed/* Lib/'' && ./Build/compile.sh`
	• Also updated containing Assets dirs' gitignores to recognize the new Editor-dir-gitignores.