nixUnstable: Update to 1.7pre3576_2b6c8ef
searx: upgrade to latest
linux: switch to 3.12 by default (latest longterm)

Close #2151.
Merge pull request #2174 from matejc/upgrade_searx

searx: upgrade to latest
ghc: add release version 7.8.1
haskell-Chart-cairo: update to version 1.2.2
haskell-Chart-diagrams: update to version 1.2.2
haskell-Chart-gtk: update to version 1.2.2
haskell-Chart: update to version 1.2.2
haskell-dyre: update to version 0.8.12
haskell-ghc-mod: update to version 4.0.2
haskell-hakyll: update to version 4.5.1.0
haskell-http-client-multipart: update to version 0.3.0.0
haskell-tasty: update to version 0.8.0.4
haskell-twitter-conduit: update to version 0.0.2.1
haskell-twitter-types: update to version 0.2.20140407
stunnel: adopt, bump to version 5.01

Signed-off-by: Austin Seipp <aseipp@pobox.com>
stdenv: Use the substitute binary instead of the shell scripted implementation
substitute: Add new package for eventual stdenv inclusion