fpm home support

[amitu]

It is common for people to work on more than one package at a time. In Git, every
repo is checked on a folder on its own, with no relationship between folders
corresponding to different git repos.

Documents in FPM package can refer to other FPM packages. So if you are working
on both packages amitu and amitu/realm for example, it is handy to checkout
amitu in a folder named amitu and the package amitu/realm in amitu/realm.

All packages in FPM should be checked out in "fpm home" folder. You can have more
than one fpm home if you so desire for whatever reason.

fpm home discovery

fpm tool expects the .fpm.ftd in a fpm home, and traverses up the folder
tree till it finds it, that folder is called fpm home.

Note: fpm home is optional. If .fpm.ftd file is not found in the expected
location it is assumed that the package is in standalone mode.

Package Folder

A package with name x/y is check-out/expected to be in $FPM_HOME/x/y.