Better detection of git/non-git repository.

Fix #498
Update ocp files for dependencies
Automatically update ~/.ocamlinit on init.

This should fix #185
Update CHANGES
Stop yelling when OPAM is not up-to-date. Now OPAM is (almost) stable, this is no so important.
Update CHANGES
Define the shell 'opam-switch-eval' function to switch and eval
Fix auto-completion scripts
Get ride of `eval` in some cases and start configuring OPAM globally.

This is still quite experimental, I need some feedback on that.

This is linked to #306 and #482
Add missing file
More work on the global configuration feature.

* rename `opam config install` to `opam config global`
* add `opam config global --info` to display the current configuration
* add
* add `opam init --no-config` to tell OPAM to *NOT* modify the global user configuration
More consistency to the global and user configuration setup.

* rename (again) the option to become: `opam config setup`
* split the options into the user ones (~/.ocamlinit, ~/.profile) and the global ones (load auto-completion and opam-switch-eval)
* meta options: `opam config setup --all`, `opam config setup --global` and `opam config --user`
* better error message when ~/.profile has already been updated by a different OPAM root
* add --dot-profile to `opam init` to select the default config file to update (default is ~/.profile)
* rename config options for `opam init` for setup: `--no-setup` and `--auto-setup`. Default behavior is asking the user.
`opam init --dot-profile=FILE` should pick as default config file given the one given by the user.
Update CHANGES
Fix message
Improve the documentation for `opam init` and `opam config setup`
Fix initialisation of ~/.profile
Shorten some message displayed to the user
The new repositories should always have the highest priority

Fix #506