Just return the value if it exists, otherwise return the default.

The guard here caused the ‘EXIT’ clause to be triggered in
`ejabberd_config:prepare_opt_val/4` on line 573 of
`ejabberd_config.erl`.
Clean up.
Add support for using yaml config file.

When converting to use the yaml config, the option for `allowed_ips`
gets stored as a binary. But needs to be compared against the client ip
of the request (which is a tuple). This converts the list of binaries
into a list of ip tuples.
Merge pull request #20 from scrogson/master

Fix mod_rest allowed_ips option to work with new yaml config.