Warn about package.el el-get conflicts
el-get-package-{files,features}: take directory

instead of package. This way we can work with non el-get packages
too (provided we can find their direectory).

* el-get-core.el (el-get-package-files, el-get-package-features): Take
  a directory instead of el-get package symbol.
* el-get.el (el-get-reload): Pass package directory instead of package
  symbol.
el-get.rcp: Delete bootstrap (M)ELPA version

Also unload the (M)ELPA package and reload the el-get installed one,
otherwise some of the defvars are not reset.
README.md: instructions for melpa install
rename el-get-autoload{s, => ing}

package.el uses <pkg-name>-autoloads.el as the destination for autoloads
generated from cookies. Meaning that during a MELPA bootstrap install
these get appended to el-get-autoloads.el. Although it's currently
harmless during bootstrap, better to avoid any possible future problems.
Merge pull request #2006 from npostavs/elpa-prior3

Improve MELPA bootstrapping