new_tag_length(): Return offset for all read data

Make new_tag_length() return an offset with the amount of data read by
it instead of one less, i.e. it returns at least an offset of '1'.
new_tag_length(): Add reference to RFC4880
Support partial body length headers
store raw packet data
Initial work on ECDSA support.
A P-256 test-case.
Corrected PGPv3 key-handling to output 64-bit keyids
Moved dumpbuffer and dumpfile helpers to __init__.py; fixed formatting of keyids to be consistent (and more legible) between PGPv3 and PGPv4 keys