Fixing multiline differentiation, fixes #77

This patch is not directly mine, it is from @drslump on issue #77 - see https://github.com/pangloss/vim-javascript/issues/77#issuecomment-18255638 for details

In testing, this change works beautifully to fix #77
Merge pull request #78 from brianarn/indent-fix-77

Fixing multiline differentiation, fixes #77