Add `AM_PROG_AR` to `configure.ac`

Required for building on the Fedora 20 toolchain.
Ignore new autotools files
Process `autoscan` feedback
Fix `scan-build` results
Ignore `make check` output
Fix nullptr dereference in arakoon_cluster_connect_master

_arakoon_cluster_node_who_master() might return successfully with
master being a NULL pointer (indicating "None"), so better check
for that before any dereferencing of this pointer.