Add reference has_log()
Extract signature conversion code
Extract string->binary conversion
Basic reflog support

For now it simply includes reading in the reflog as a list of
entries. This does mean that the drop/append functions don't need to be
wrapped, as we can use erlang's list handling.

It however also means that we cannot write this back out, as there is
currently no way to create an empty reflog that we can fill with our own
list.
Correct the signature output from the reflog

The function to convert signatures into erlang terms was returning ok
with the rest of the tuple which geef_sig expects. We do not want the
ok there, so let that function give us the four terms and decide what to
do in each case.

The signature functions still want to return with ok before it, but the
reflog wants them together with the other parameters.
Add geef_reflog:delete()
Merge branch 'reflog'