doc for binding to unix domain socket

If host contains a `/` then it is assumed to be a path
to a unix domain socket and the server will bind to that
instead of using TCP

It will also set `server.info.unixDomainSocket` to the
absolute path to the socket.
add failing test for using unix domain socket
implement ability to listen on unix domain socket

If server option `host` contains a `/` then it is assumed
to be a unix domain socket and the server will listen on it
rather than TCP.

`server.info.unixDomainSocket` will contain the absolute path
to the socket it is listening on.
Initial internal move to joi 2.0
Migrate internal schema validation to joi 2.0
version switch
1.16.0
1.16.0
Merge pull request #1133 from spumko/issue/1132

Joi 2.0
Update joi schema usage
Merge pull request #1123 from jeffbski/unixDomainSocket

add ability to listen on unix domain socket
Cleanup for #1123
Windows named pipes. Closes #1129
Add ability to set joi validation globals
handlebars 1.1.x. Closes #1136
Remove <p>. Closes #1135
Fix request._route override. Closes #1137
Fix ext function plugin env binding
Merge pull request #1142 from spumko/issue/1138

Fix ext function plugin env binding
Improve schema validation and docs