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
Added OCaml SAT solver

Currently this isn't connected up to anything.
Fixed copying of Python files to dist directory
Enable delegating implementation fetching to an external helper
Added support for getting preferred languages
Added version expressions
Default to installing in /usr

Python doesn't look in /usr/local by default on all systems, so
installing there might not work.
Disable multiarch support on Linux if /lib/ld-linux.so.2 is missing

Otherwise, we try to run 32-bit binaries and get a useless error
message.
Don't abort solving just because a local feed is missing

Just log the "Missing local feed; if it's no longer required, remove it with"
message instead.
Moved class distro from General to Options

Otherwise, everything depends on the Distro module, which
causes problems with circular dependencies.
Added try_cleanup_distro_version
Enable coverage testing
Added Versions.dummy

This is a magic version that matches every restriction. The solver needs
this for the dummy implementations it adds when generating diagnostics.
Delay initialising the distribution until we need it
Improved feed parsing

We now extract commands, dependencies, restrictions and stability
levels.
Added system.platform

OCaml doesn't provide a uname system call, so we have to spawn a
subprocess for this.
Process <package-implementation> elements

Currently only implemented for Arch Linux.
Allow attaching user-defined data to SAT variables

Also, added a quick way to set all remaining variables to False.
Accept old-style digest-in-id in selections
Initial 0install solver

This uses the feeds to construct a SAT problem, solves it, and then
builds a selections document from the result. The core functions are
working, but many features are still missing.