Preliminary support for OSX

Hi there,

I just started an attempt at being able to use devassistant on OSX, using Homebrew as package manager.
In the current state, it works with dedicated assistants (i.e. I did not yet add all the 'homebre' dependencies in the assistants) and 2 tests are failing (but it seems they're failing as well on my docker container).

A preliminary dependency is `sed` (i.e. `brew install sed --default-names`) as the assistants are written with the GNU sed in mind.
Note also that in some cases I do think the assistants need to be reviewed (i.e. regarding rails for example, it is considered as bad practice to install the platform version. People are usually installing the needed rails version locally thanks to `bundler`)

Regards