versioning: include an "appname" in the application version string in the versioning protocol, and make that appname be controlled by setup.py

It is currently hardcoded in setup.py to be 'allmydata-tahoe'.  Ticket #556 is to make it configurable by a runtime command-line argument to setup.py: "--appname=foo", but I suddenly wondered if we really wanted that and at the same time realized that we don't need that for tahoe-1.3.0 release, so this patch just hardcodes it in setup.py.
setup.py inspects a file named 'src/allmydata/_appname.py' and assert that it contains the string "__appname__ = 'allmydata-tahoe'", and creates it if it isn't already present.  src/allmydata/__init__.py import _appname and reads __appname__ from it.  The rest of the Python code imports allmydata and inspects "allmydata.__appname__", although actually every use it uses "allmydata.__full_version__" instead, where "allmydata.__full_version__" is created in src/allmydata/__init__.py to be:

__full_version__ = __appname + '-' + str(__version__).

All the code that emits an "application version string" when describing what version of a protocol it supports (introducer server, storage server, upload helper), or when describing itself in general (introducer client), usese allmydata.__full_version__.

This fixes ticket #556 at least well enough for tahoe-1.3.0 release.

src/allmydata/client.py

@@ -115,7 +115,7 @@ class Client(node.Node, pollmixin.PollMixin):
         self.introducer_furl = self.get_config("client", "introducer.furl")
         ic = IntroducerClient(self.tub, self.introducer_furl,
                               self.nickname,
-                              str(allmydata.__version__),
+                              str(allmydata.__full_version__),
                               str(self.OLDEST_SUPPORTED_VERSION))
         self.introducer_client = ic
         # hold off on starting the IntroducerClient until our tub has been