Merging bundle-el into el-get 

As we discussed in #1967, I would like to merge my [bundle-el](https://github.com/tarao/bundle-el) to el-get.

- `bundle.el` ~~and `eval-after-load-compile.el`~~ will be merged as `el-get-bundle.el` ~~and `el-get-eval-after-load-compile.el` respectively~~
- Related functions are added to `el-get.el` as `autoloads` (They are not `require`d since they are not needed by the el-get's core functionality)
- No documentation is provided: I'm not sure where to add
    - The simplest way is to copy [the original one](https://github.com/tarao/bundle-el/blob/el-get/README.md) to el-get's README?
Style nitpick: it would be nicer to make this list with a quote, when I see `list` I expect there will be some computation. That does screw up the indentation a bit, but that may be solved by using an alist instead of a plist.