initial empty check-in
README: introduction explaining the projet aims
README: addition of a few implementation details
Addition of primitives for SHA-1 hashing and HMAC

sha1.h/c contains the actual SHA-1 code (RFC 3174)
sha-test.c contains tests of SHA-1 and HMAC-SHA-1 (RFC 3174 and 2202)
Basic message encoding and decoding
Addition of tools for buffers, dynamic arrays and S-expressions
First draft of the client main code
Addition of small utility functions

- prototype for a message logging function
- get_mtime()
First draft of the server main code
Implementation of a log to stderr
Makefile update to build the client and the server
Server: automatic reload of the configuration file on change
Server: checking message address against peer address
Server: checking message time
Server: addition of account up and down events
Server: keeping track of last seen address, to update only when needed
Addition of a TODO list
S-expression library switch and compilation option change

 - The old struct buf based S-expression library has been replaced by my
   stand-alone C-string-based S-expression library, from `codemod` repository
   (commit 4dd74891e025a00fddaf313724a5ad810498321a).

 - Stricter compilation options are used, a few source lines have been
   changed accordingly.
csexp: bugfix import

 - update to csexp from codemod commit 1330dd09878d43dfb55cf6df99883df064a1c496
   (bugfix for S-expression duplication)
Client: support of self address discovery

 - sensor.o is a new module for self IP address determination
 - currently the only support method is popen()ing a subprocess