add TCP User Timeout Option (RFC 5482)

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

Allow L2pcapListenSocket to return None if where is no packets (issue #74)

Allow the use of file-like objects with wrpcap() (and (Raw)PcapWriter())

IKEv2: fix fields overload (fixes #79)

Allow one to send the TCP User Timeout Option described in RFC 5482 in
a crafted TCP segment.

Allow L2pcapListenSocket.recv() to return None if where is no captured packets. Make sniff() to continue loop if received pkt is None (issue #74)

Move the Pcap/PcapNg detection from rdpcap to (Raw)PcapReader

Remove unneeded tests

Reported by Aaron Wood on Scapy ML.

This does not change the use of rdpcap() with both formats, and allows
using (Raw)PcapReader() against a PcapNg files.

Multiple ASN.1 enhancements ; X.509 comprehensive support 

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

Heavy modifications to x509.py : comprehensive support for many more fields, extensions and keys (both RSA and ECDSA)

Heavy modifications to asn1fields.py : proper and exhaustive support for ASN1 objects as fields (SEQUENCE, CHOICE, _optional, etc.)

Add implicit/explicit tagging support ; add high-tag number support ; add more proper BER bit string encoding/decoding