added offloaded key generation

this adds a new service to pre-generate RSA key pairs.  This allows
the expensive (i.e. slow) key generation to be placed into a process
outside the node, so that the node's reactor will not block when it
needs a key pair, but instead can retrieve them from a pool of already
generated key pairs in the key-generator service.

it adds a tahoe create-key-generator command which initialises an 
empty dir with a tahoe-key-generator.tac file which can then be run
via twistd.  it stashes its .pem and portnum for furl stability and
writes the furl of the key gen service to key_generator.furl, also
printing it to stdout.

by placing a key_generator.furl file into the nodes config directory
(e.g. ~/.tahoe) a node will attempt to connect to such a service, and
will use that when creating mutable files (i.e. directories) whenever
possible.  if the keygen service is unavailable, it will perform the
key generation locally instead, as before.

src/allmydata/scripts/runner.py

@@ -4,9 +4,13 @@ from cStringIO import StringIO
 from twisted.python import usage
 
 from allmydata.scripts.common import BaseOptions
-import debug, create_node, startstop_node, cli
+import debug, create_node, startstop_node, cli, keygen
 
-_general_commands = create_node.subCommands + debug.subCommands + cli.subCommands
+_general_commands = ( create_node.subCommands
+                    + keygen.subCommands
+                    + debug.subCommands
+                    + cli.subCommands
+                    )
 
 class Options(BaseOptions, usage.Options):
     synopsis = "Usage:  tahoe <command> [command options]"
@@ -60,6 +64,8 @@ def runner(argv, run_by_human=True, stdout=sys.stdout, stderr=sys.stderr,
         rc = debug.dispatch[command](so, stdout, stderr)
     elif command in cli.dispatch:
         rc = cli.dispatch[command](so, stdout, stderr)
+    elif command in keygen.dispatch:
+        rc = keygen.dispatch[command](so, stdout, stderr)
     elif command in ac_dispatch:
         rc = ac_dispatch[command](so, stdout, stderr)
     else: