separate file visitor and file hashing
disable sandbox test

was creating an useless "toCreate" file in current dir
check for illegal characters in path

test is not really exhaustive, and we don't have any common definition
of "illegal characters" shared with entries.
remove values already filtered at path level
rename File{Hash,Digest}Visitor
update readme
remove few comments not necessary anymore
remove useless test
fix memory store read-only usable space

unallocated/total space of store have to be consistent with free/total
space available in store, however, "available" space must be zero when
the store is read-only since we can't write anything to it
disabled test & notes about storage capacity
code reformat