* recipes/org-tree-slide.rcp: new recipe.

test-recipe.sh
```
org-tree-slide installed: This package has been installed successfully by el-get.
el-get install org-tree-slide
*** Second install successful ***
*** SUCCESS ../recipes/org-tree-slide.rcp ***
```

M-x el-get-check-recipe
```
0 error(s) found.
```