Add a SHA1 implementation

Add an implementation of the SHA1 hash algorithm based on a public
domain project by Steve Reid <sreid@sea-to-sky.net>. You can get the
original sources from:

    ftp://ftp.funet.fi/pub/crypt/hash/sha/sha1.c

We will need this for several things. File names of note files will be
generated based on a hash their content instead of using a random name.
Items can be uniquely identified using a hash.

In addition to the regular sha1_init(), sha1_update() and sha1_final()
operations, our implementation also contains wrappers for hashing a
string or a stream.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/Makefile.am

@@ -8,6 +8,7 @@ calcurse_SOURCES = \
 	htable.h \
 	llist.h \
 	llist_ts.h \
+	sha1.h \
 	apoint.c \
 	args.c \
 	calendar.c \
@@ -22,6 +23,7 @@ calcurse_SOURCES = \
 	note.c \
 	notify.c \
 	recur.c \
+	sha1.c \
 	sigs.c \
 	todo.c \
 	utf8.c \