Add a HMAC implementation based on top of APR's SHA1.

No unit-tests yet, but conforms to the HMAC-SHA1 Spec
false security is false, might as well return void
get rid of APRX prefix
Merge branch 'hmac' (see issue #17).
Merge remote-tracking branch 'upstream/master'

Conflicts:
	Makefile
	src/cookie.c
	src/hmac.c
	src/hmac.h