install assets as relative symlinks

it will fallback to absolute symlinks or hard copies since symfony 2.6
feature #718 install assets as symlink (Tobion)

This PR was merged into the 2.6-dev branch.

Discussion
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install assets as symlink

This is now safe because it will fallback to hard copies since symfony 2.6.

Commits
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3b5f195 install assets as relative symlinks