Ignore build, MacOSX and Windows files

With the current setup these folders are ignored in the whole repo, but they're only automatically generated in the project root so it makes sense to only ignore the ones in the root.

I made this change because one of the assets in my project uses a folder called 'Library', and I was wondering why it never uploaded that folder.

And removes the last new line.

Removes queried extensions.

Adds several annoying files to Unity3D ignore file, like platform cache files (Mac + Windows), the famous Unity3D pidb.meta

By having the leading `/` infront of the `*` makes these types of files relative to the directory the gitignore is in. If you create a Unity project inside of a git directory Unity forces you to create a new directory, thus not picking up these files.

Ignore the build directory in case a Unity build is initiated in the project.

Sysinfo.txt is generated by unity if occurs any crashes. The content consist of  users system information, running process etc. I don't think that should be shared with others.

Unity only generates project files in the top-level directory.
However, the current .gitignore will ignore all project files even
if they were not generated by Unity.

It is sometimes necessary to include non-Unity project files for
level editors or other utility projects that have been added to the
repository.

*.suo files are binary, user specific files generated by Visual Studio with some user configuration information, thus, don't need to be versioned.