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?
Fixed in e8a055d5aad33297fb2ab8e4def5bfc2c1700790.

There was a difference in `el-get-byte-compile-file` that it always ignores compilation warnings.  I think compilation warnings are useful information for writing user emacs file.  I have changed it to have an optional parameter to enable the warnings in f2c9d0fdfacc04c0044b7ddbf856b63c5e1efc26. (The Default is `nil` - no warning.) `el-get-bundle-load-init` uses this with passing `byte-compile-warnings` so that a user have a chance to disable/customize compilation warnings.