Updated comments replaced relcool with relx

Updated comments replaced relcool with relx
Updated comments replaced relcool with relx

Updated comments replaced relcool with relx
Merge pull request #91 from byaruhaf/patch-1

Updated comments replaced relcool with relx
Merge pull request #92 from byaruhaf/patch-2

Updated comments replaced relcool with relx
Properly detect the default release and fail on conflicts

Closes #93
Merge pull request #95 from josevalim/jv-default-release

Properly detect the default release and fail on conflicts
Add option to skip release discovery

`rlx_prv_discover` provider recursively scans all directories in
`lib_dirs` list looking for `*.app` and `*.rel` files.

This commit adds an option `disable_rel_discovery` to skip searching for
`*.rel` files when we know there aren't any. This speeds up release
generation with a lot of dependencies on the `libs_dir` list
significantly and reduces total release generation time in half.