Alibuild

* Refactor code and support for the pb multicommand
* Improve arch detection in alienv
* Add proper Unicode support for both Python 2 and 3
* Fix bug causing undesired rebuilds on Python 3
* Keep recipe ordering of environment variables
* Add proper relocation on macOS
* Improve support for defaults

* Refactor code and support for the `pb` multicommand
* Improve arch detection in `alienv`
* Add proper Unicode support for both Python 2 and 3
* Fix bug causing undesired rebuilds on Python 3
* Keep recipe ordering of environment variables
* Add proper relocation on macOS
* Improve support for defaults

No need to sudo on macOS.

People using the Git repository directory might have to install PyYAML now, since aliBuild does not use it's own copy to guarantee compatibility with Python 3.

(no need to sudo on mac)

People using the git repository directory might have to install PyYAML now, since aliBuild does not use it's own copy to guarantee compatibility with python 3.

New features & bug fixes:
- Python 3.x support: you can now use aliBuild with python 3.
- Improved supports for defaults. You can now use defaults to change all of the recipe metadata, for example the version of a given tool. This is detailed in http://alisw.github.io/alibuild/user.html#defaults. Most notably, alice users can use `--defaults root6` to use ROOT6 or `--defaults o2-daq` to have a minimal configuration when compiling AliceO2.
- Improved `aliDoctor`: now behaves exactly like aliBuild and supports the `--defaults`, `--docker` and `--disable` option.
- Improved docker support: major bug fixes and the ability to specify your own image via the newly introduced `--docker-image` option.
- Improved support for multiple development areas. aliBuild clean should now be consistent when cleaning up things.

Notable changes:
- **Experimental support for python3**: aliBuild should now work correctly when running in a python3 environment.
- **Extended defaults support**: it's now possible to change metadata of packages and enable / disable them from the so called [defaults file](https://alisw.github.io/alibuild/user.html#defaults). This means, for example, that there is no need anymore for a separate `root6` branch in alidist, as you can simply use `--defaults root6`.
- **Reworked internals**: some of aliBuild internals have been reworked to increase code reuse and improve testing.
- Usual round of bug fixes. Full history of changes at: https://github.com/alisw/alibuild/compare/v1.3.1...641a98b858e58f6966e42ee8c848dc28e6fc0723