Switched from the old haskell HTTP library to http-conduit.

The hoary old HTTP library was only used when checking if an url exists,
when curl was not available. It had many problems, including not supporting
https at all.

Now, this is done using http-conduit for all urls that it supports. Falls
back to curl for any url that http-conduit doesn't like (probably ftp etc,
but could also be an url that its parser chokes on for whatever reason).

This adds a new dependency on http-conduit, but webdav support already
indirectly depended on that, and the s3-aws branch also uses it.

This opens up the possibility of using http-conduit for large file
downloads, but for now I've left it using wget/curl.

This commit was sponsored by Paul Tötterman.

git-annex.cabal

@@ -96,11 +96,11 @@ Executable git-annex
   Main-Is: git-annex.hs
   Build-Depends: MissingH, hslogger, directory, filepath,
    containers, utf8-string, network (>= 2.0), mtl (>= 2),
-   bytestring, old-locale, time, HTTP, dataenc, SHA, process, json,
+   bytestring, old-locale, time, dataenc, SHA, process, json,
    base (>= 4.5 && < 4.9), monad-control, exceptions (>= 0.5), transformers,
    IfElse, text, QuickCheck >= 2.1, bloomfilter, edit-distance, process,
    SafeSemaphore, uuid, random, dlist, unix-compat, async, stm (>= 2.3),
-   data-default, case-insensitive
+   data-default, case-insensitive, http-conduit, http-types
   CC-Options: -Wall
   GHC-Options: -Wall
   Extensions: PackageImports
@@ -141,8 +141,7 @@ Executable git-annex
     CPP-Options: -DWITH_S3
 
   if flag(WebDAV)
-    Build-Depends: DAV (>= 1.0),
-     http-client, http-types
+    Build-Depends: DAV (>= 1.0), http-client
     CPP-Options: -DWITH_WEBDAV
 
   if flag(Assistant) && ! os(solaris)
@@ -188,7 +187,7 @@ Executable git-annex
   if flag(Webapp)
     Build-Depends:
      yesod, yesod-default, yesod-static, yesod-form, yesod-core,
-     http-types, wai, wai-extra, warp, warp-tls,
+     wai, wai-extra, warp, warp-tls,
      blaze-builder, crypto-api, hamlet, clientsession,
      template-haskell, data-default, aeson, path-pieces,
      shakespeare