Framework.py split: New pelix.internals package

Service registry, constants, exceptions and beans are now in separate
files in the pelix.internals package.

This reduces the size of the pelix.framework module, and should ease
future developments
iPOPO core split into multiple modules

Code is now more readable
Shell command ipopo.instance prints the required specifications
Merged Pelix exceptions and constants in pelix.constants

More logical place to find them.
Avoid modifying Future protected fields directly

Added an execute() method in Future that updates its own state
Some updates according to pylint

- Code format update
- Some variable could have not been initialized by loops