enhance version management

When making an official release of scapy, one must:

* Modify the hardcoded version in setup.py and scapy/config.py
* Create a commit with this change
* Create a git tag with the same version
* Run ./setup.py sdist register upload
* Modify the hardcoded version in setup.py and scapy/config.py again to
  append '-dev'.
* Create another commit with the modified version.
* Push the 3 commits and the tag.

Not only this is tedious but it is also error prone.

Add utility functions in scapy/__init__.py to determine current version.

If git is available (thus running from a git clone), use "git describe" to
get the current version. Write the version for future reference to a
text file (scapy/VERSION).

If git is not available (running from an installed scapy package) read
the version from the scapy/VERSION file.

This changes the release process as follows:

* Create a git tag on the commit where you want to release
* Run ./setup.py sdist register upload
* Push the tag

This allows to have a single place where the version is managed: git.

Note: change the development versions to X.Y.Z.devN where N is the
number of commits after the last tag. This complies to PEP 440
https://www.python.org/dev/peps/pep-0440/#developmental-releases



Signed-off-by: Robin Jarry <robin.jarry@6wind.com>