FIX: fix_version enumeration ordering

Make "enum fix_version" enumeration ordered by protocol versions in
ascending order so that you can use comparison operators for version
specific logic.

NOTE! This breaks ABI compatibility for applications built against older
versions of libtrading.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
FIX: Fix EndSeqNo for FIX 4.1 and earlier

The EndSeqNo is specified as follows for FIX 4.1 and earlier:

  Message sequence number of last record in range to be resent. If
  request is for a single record BeginSeqNo = EndSeqNo. If request is for
  all messages subsequent to a particular message, EndSeqNo = "999999"

Fixes #49.

Signed-off-by: Pekka Enberg <penberg@iki.fi>