Install script: Improved requirements checking

These are the points that were improved in this version :
  - Refined message() to implement it as a logger-like function
  - Testing cURL as downloader if wget cannot be found
  - Testing Python interpreter passed as an environment variable
  - Iterating through PATH to find a compatible Python interpreter
  - Calling PIP directly from the found Python interpreter
Merge remote-tracking branch 'origin/install-script' into install-script

Conflicts:
	tryton/gnuhealth_install.sh