Move custom build scripts to a separate directory

[snowleopard]

As discussed in #473, with the increasing number of various custom build scripts, it becomes confusing for users to understand which one they should pick.

Perhaps we should only keep the default build.sh and build.bat at the top level and create a separate directory called scripts for custom build scripts?

Probably a good idea yes. With a short README that says "use this one if you want to build hadrian using X" for each specific script.

So, here is a tentative plan:

• Move custom build scripts to subdirectory script.
• Add script/README.md with a short description of each build script.

P.S.: An alternative name for the subdirectory could be build. We'll then have:

• hadrian/build.sh is default for Linux/OS X
• hadrian/build.bad is default for Windows
• hadrian/build/cabal.sh -- using Cabal on Linux/OS X
• hadrian/build/stack.bat -- using Stack on Windows
• hadrian/build/nix.sh -- for building with nix
• etc.