Import original tensor.md

Copied from:
curl -O https://raw.github.com/torch/torch7/680c2994ce381d22dc597a5eecd8f28f50a305b9/doc/README.md
Add header
Respect GHFM line-break before code block

Otherwise Redcarpet is lost.
Fix style to allow self-closing anchors

Thanks to the following poster!
http://forums.devshed.com/css-help-116/css-and-self-closing-anchor-tags-87992.html
Fix link
Make it more concise and remind of committing
Merge branch 'master' into gh-pages