When using scapy as a third party library, we do not "benefit" from the
loading routine that puts all symbols in the __builtin__ namespace. This
implies that all modules must be "importable" without any errors for
undefined symbols.

This patch fixes importing every scapy module individually. Some
circular imports were fixed (between inet6 and dhcp6).

Most errors were detected by using pylint with the right configuration
(scapy produces a *lot* of errors). Other errors were detected with the
following shell script:

    export PYTHONPATH=".:$PYTHONPATH"
    for f in $(find scapy/ -name '*.py'); do
        f=${f%/__init__.py}
        module=$(echo ${f%.py} | sed 's,/,.,g')
        python -c "import $module; print $module"
    done

Also, replace all implicit relative imports by absolute imports to avoid
surprises with some python versions.

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

cert.py improvements

* Improve cert.py : independence from openssl, ECDSA support, sign/verify capabilities

* Make travis install python-ecdsa

* Clarify ECDSA support

* Cleaning up for cert.py

EAPOL / EAP updates and regression tests

* Flip conditional for simplicity
* Indent lines
* Include Travis CI build status image & link

Allow one to send the TCP User Timeout Option described in RFC 5482 in
a crafted TCP segment.
Also added unit tests in new "TCP options" section of
test/regression.uts

Insert DER test data inside x509.uts and move it from scapy/layers/ to test/ (also, remove obsolete X.509 tests from test/regression.uts

As suggested by @guedou, this adds tests for fragment() and
defragment() functions.