Added a preliminary "gf -server" mode.

The command "gf -server" now starts a simple HTTP server on port 41295,
providing a simple web API to the GF compiler. It currently support the
follwing operations:

  * creating new temporary directories for grammar uploads,
  * uploading grammars files for use in the GF shell,
  * executing GF shell commands, and
  * accessing static files.

This means that GF now depends on some additional networking related packages,
but they should be available and easy to install on all platforms. There is
also a new configuration flag "server" in gf.cabal, so GF will be compiled
without support for server mode if the extra packages are unavailable.

Note that running gf -server while connected to the internet can be a security
risk. To prevent unauthorized access to the rest of the system, it is
advisable to run the server in GF_RESTRICTED mode and as a user with suitably
restricted file permissions.

src/compiler/GF.hs

@@ -45,5 +45,5 @@ mainOpts opts files =
       ModeHelp        -> putStrLn helpMessage
       ModeInteractive -> mainGFI opts files
       ModeRun         -> mainRunGFI opts files
+      ModeServer      -> mainServerGFI opts files
       ModeCompiler    -> dieIOE (mainGFC opts files)
-