Build instructions for Windows
Improved error handling in check_output
Unit tests for tab completion
Fixed a TODO error in tab completion
Fixed "0install add" completion
Deleted Python completion code

A few tests had to be commented out as they are part of other tests
which are still in the Python. However, there's no point testing the
Python version of the completion, since it's never used now anyway.
Spawn background updates from OCaml

If an app needs to be updated in the background, fork a background
Python process to do that while execing the current selections
immediately. Before, we let the Python do the fork, which is slower
and also didn't work on Windows.
Fixed spawn_detach on Windows
Handle "0install --help" in the OCaml
Handle --help for all commands in the OCaml

The output is identical to the Python for all commands, except that:

- The Python incorrectly gave a --xml option for "add"
- The Python incorrectly missed --offline for "import" (to prevent
  downloading keys)
- Some meta vars have been renamed for clarity (e.g. --main=MAIN becomes
  --main=PATH).
- The order of the options is different.
Trim whitespace inside <arg> elements

It seems that some versions of Python add newlines here:

http://thread.gmane.org/gmane.comp.file-systems.zero-install.devel/7074
Don't make a _build symlink if it exists as a directory

Reported by Tim Cuthbertson.
Converted "0install show" to OCaml

The Python version has been removed. The unit-tests now run the OCaml
version.
Implemented --dry-run in OCaml
Parsing <binding> and <overlay>

Attempting to run a selections document containing these logs a warning.
Added version parsing

This allows version completions to sort the results correctly.
Converted "0install select" to OCaml

Note that this is just the parsing and output parts. Downloading and
solving are still delegated to the Python.
Added realpath implementation
Fixed "make share/locale/zero-install.pot"

intltool-extract doesn't like absolute paths.
List.iteri is only available in OCaml >= 4