Linux Port

Porting `auto-update` to linux induces a few complications & tradeofs such as:
- There's no such things as MacOS packages
- Finding the executable path is non-portable cross unix flavours ([stackoverflow](http://stackoverflow.com/questions/1023306/finding-current-executables-path-without-proc-self-exe))
- Most linux distributions have package managers that update software automatically

After a discussion on #chromium, it looks like chrome on linux lets the package manager update the distributed software and notifies the user in the UI whenever a new version is installed, prompting her to relaunch chrome.
## Chrome approach
- Let the package manager handle updates
- Provide instructions to construct auto-update compliant npm, deb packages
- Build a library to detect updates from the package manager prompting the user to relaunch
## Independent approach
- We would have to enforce the fact that software release would have to be packaged in a directory easily identifiable from the running executable path.
- We would want to avoid prompting a root password anytime the program tries to update itself. auto-update managed package would probably have to set permissions in a quite specific way at install to support the update model: eg. setgid on the exec and authorize write for the given group on all relevant files

It guess this deserves a discussion here to see what would work best!

No files, tests, or entry points are named. Start by reviewing the proposed Chrome/package-manager approach against the independent approach, then define an agreed Linux update strategy; the issue is done when that direction and its implementation scope are decided.

Written by the indexing model from the issue text.