after install have a xqerl greeter

[grantmacken]

• add supervisor and server code for loading xquery main modules in priv dir
• add main module 'greeter.xq` to priv/modules dir
• add associated css and image files to priv/static/assets dir
• alter cowboy routes so http://localhost:8081/xqerl will display greeter page
• write check in actions to show greeter is working

Yep I know I could have chucked a index.html file into /priv/static/assets and let the cowboy static assets handler do its thing.
however I want to use the priv/modules dir for other stuff so I did it this way.

[grantmacken]

xqerl always listens on port 8081.
so after installing xqerl there could be a greeter on http://localhost:8081
to show the install has been completed successfuly.

Prior practice: Greeters on port 80 after installing nginx or openresty.
example:

Welcome to OpenResty!!

If you see this page, the OpenResty web platform is successfully installed and working. 
Further configuration is required.

For 0nline documentation and support please refer to openresty.org.

Thank you for flying OpenResty.

https://welcome.vgs.lenovo.com.cn/

[zadean]

This is a really good idea.

I think a default greeter should be overridable by a user XQuery module. Then it's there, unless it shouldn't be. right?

[grantmacken]

Pr #52 merged