Allow mounting a path in a different location in the chroot

Fixes #24.
nix-build: Support talking to old daemons

Fixes #76.
Add a test for incomplete closures in the binary cache

Issue #77.
Automatically fall back if the references of a substitute are not substitutable

Fixes #77.
If a substitute closure is incomplete, build dependencies, then retry the substituter

Issue #77.
Update release notes
Use sysconfdir=/etc
Reinstate the http://nixos.org/binary-cache default for the binary-caches setting
Manual: Fix "nix-store --export" example
Urgggh

http://hydra.nixos.org/build/3661100
Clear any immutable bits in the Nix store

Doing this once makes subsequent operations like garbage collecting
more efficient since we don't have to call makeMutable() first.
Remove tabs
Open the database after removing immutable bits
Fix "0 store paths deleted" message
Delete a left-over trash directory before doing a GC
Bump version number to 1.4
Store build logs in /nix/var/log/nix/drvs/<XX>

...where <XX> is the first two characters of the derivation.
Otherwise /nix/var/log/nix/drvs may become so large that we run into
all sorts of weird filesystem limits/inefficiences.  For instance,
ext3/ext4 filesystems will barf with "ext4_dx_add_entry:1551:
Directory index full!" once you hit a few million files.
corepkgs/fetchurl: Enable making the downloaded file executable
build-remote.pl: Don't keep build logs on the build slave
Fix the VM tests