Config v2

[LincolnBryant]

new config format along with a new organizational structure:

# ebb minimal configuration.
# unknown keys are rejected

[dhcp]

[[dhcp.subnet]]
cidr = "172.16.0.0/12"
interface = "pxe0"

[[dhcp.subnet.pool]]
range = "172.16.0.0/12"

The configuration also supports [[dhcp.proxy]] as I anticipate needing proxyDHCP.

There's a few pieces to note..

1. Added a new config validator (ebb_config_validator.erl), defining a fixed schema, which takes a structural pass over the config file, and rejects unknown keys
   • this does NOT do semantic validation. that can come later as a second pass
2. The dhcp supervisor spawns a supervisor for each subnet, and each subnet supervisor spawns a dhcpd binding to port 67, and pool server(s) for each subnet that handle given ranges
   • thus, interfaces are bound per subnet, and multiple pools can exist per subnet with different configurations (lease times, ranges, backend implementation)
3. changes the default log format to be a bit more syslog friendly

this PR closes #5