[3080] Initial PostgreSQL patch

 - patch contributed by David Carlier (thanks!)
 - updated to current master
 - fixed compliation issues
 - written developer's guide (pgsql setup steps)
 - many smaller changes
 - still WIP
[3080] IPv4 addresses are now properly stored in integer fields
[3360] Set the lease file location for the Memfile backend.
[3080] Support for fqdn_fwd, fqdn_rev, hostname fields + fixes

 - Added support for 3 new fields, altered schema
 - Fixed bug where empty HWADDR was throwing exception
 - Several clean-ups
[3080] PgSql improvements:

 - UPDATE_LEASE6 query is now working
 - Added bunch of new unit-tests
[3080] MySQL unit-tests reordered (grouped in v4 and v6 sections)
[3080] Several queries fixed (now use lease_type parameter)
[3360] Implemented base class for read/write in CSV file.
[3080] BIND10 Guide updated with Postgres support.
[3360] Added CSV file validation.
[3080] Many comments added for PostgreSQL backend
[3360] Implemented class for writing/reading DHCPv6 leases from CSV.
[3360] Implemented CSV parser for DHCPv6 leases.
[3360] Cleanup the CSV file parser for DHCPv6 leases.
[3360] Implemented the CSV lease file parser for DHCPv4.
[3360] Memfile stores and reads leases from disk for v4 and v6.
[3360] Memfile db access string uses persist parameter to disable writes.
[3360] Updated description of the Memfile backend class.
[3360] Cleanup in the Memfile backend code.
[3360] Memfile now opens v4 or v6 lease file, not both.