Replace `build.sh` and `build.cmd` with `build.d` ...

Since dub needs a D compiler anyway, we might follow in DMD's step and use D for building,
as it is more reliable and expressive than a shell script as features pile up.

This unifies the two scripts to expose the same set of functionalities on all platforms,
with a behavior that is hopefully more sensible
(tradeoff and reasoning are explained through the code / documentation).
It is also completely independent from the current working directory.

Some discrepancies included:
- Name of the variable to override to select the compiler (DC vs DMD);
- Version file: Windows was always regenerating it;
- Version file: Windows would display "unknown" if `dub describe` failed,
  POSIX would use an empty string;
- Compiler detection: POSIX would detect ldmd2 / gdmd if dmd is not present,
  Windows would just fail;

All those functionalities are now unified in a single script.
The old script are kept for compatibility, but forward to the new script,
and can be removed in a few releases.

Revise build.cmd (remove curl.lib, add -O) ...

-O is used in build.sh; the curl (import) library isn't required anymore
since Phobos v2.069.

implement $DUB build variable ...

fix localized import


do not escape DUB path

DUB_EXE in script environment

The default compiler should not depend on DUB's own build environment

Use the actual binary name used to build DUB as the default compiler. See #303.

Don't use "echo off" to avoid influencing potential parent batch files.

Add a DUB configuration to compile as a library without curl support.

Remove -property flag from build scripts.

Module statement addded to autogenerated version_.d

Query git to determine the current DUB version and report it on the help screen. Fixes #39.

Changed the windows build script to directly call the compiler instead of rdmd. ...

The compiler used can be customized by setting the DC environment variable ("dmd" is the default and "gdmd" or "ldmd" are the most likely alternatives).

Don't print build success after build errors.

Improve wording of the post-build message.

dub will be build into the bin directory, so that when the dub executable's location is added to the path, the build.cmd / build.sh is not made globally accesible.

Add -w -property to the build scripts and fix the build appropriately.

Added files necessary to build on Windows.