introduce new tool 'bee fetch'

this tool allows us to install bee packages from remote repositories

* add URL(s) (or directories) to the file ${XDG_CONFIG_HOME}/bee/beeremote
 (one URL per line; comments beginning with '#')

* execute 'bee fetch' to get all information about remote bee packages from the server(s)

* execute 'bee install' for the desired remote package(s) to install
 (the bee package will be retrieved and installed from the remote repository)

* 'bee list' also lists available remote packages

* remote repositories must contain a file 'pkglist' which contains 'ls *bee*'
 (one package per line)