Added a separate HEAD command

If you're PUTting large quantities of entities over a network connection, you
may not want to wait for a round-trip for every entity, hence the "test whether
an entity exists" functionality belongs under a separate verb.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added SHA-1 calculation code

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Makefile.am: Remove unneeded AM_CXXFLAGS

This is not a C++ project.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Got basic functionality working

GET, PUT and HEAD seems to be working adequately now.  There's no pack-file
generation at this point.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>