add mac native build

add mac native build

This patch adds support for a mac native build.

At the moment it's a fairly simple .app - i.e. so simple as to be unacceptable
for a shipping product, but ok for testing and experiment at this point.

notably once launched, the app's ui does not respond at all, although its dock
icon does allow it to be force-quit.

this produces a single .app bundle, which when run will look for a node basedir
in ~/.tahoe.  If one is not found, one will be created in ~/Library/Application
Support/Allmydata Tahoe, and that will be symlinked to ~/.tahoe

if the basedir is lacking basic config (introducer.furl and root_dir.cap) then
the wx config wizard will be launched to log into an account and to set up
those files.

if a webport file is not found, the default value of 8123 will be written into
it.

once the node has started running, a webbrowser will be opened to the webish
interface at the users root_dir

note that, once configured, the node runs as the main thread of the .app,
no daemonisation is done, twistd is not involved.

the binary itself, from within the .app bundle, i.e.
"Allmydata Tahoe.app/Contents/MacOS/Allmydata Tahoe"
can be used from the command line and functions as the 'tahoe' executable
would in a unix environment, with one exception - when launched with no args
it triggers the default behaviour of running a node, and if necessary config
wizard, as if the user had launched the .app

one other gotcha to be aware of is that symlinking to this binary from some
other place in ones $PATH will most likely not work. when I tried this,
something - wx I believe - exploded, since it seems to use argv[0] to figure
out where necessary libraries reside and fails if argv[0] isn't in the .app
bundle.  it's pretty easy to set up a script a la
    #!/bin/bash
    /Blah/blah/blah/Allmydata\ Tahoe.app/Contents/MacOS/Allmydata\ Tahoe "${@}"