Add IntroducerClient write-only yaml cache file

this change also includes unit tests

src/allmydata/_auto_deps.py

@@ -87,6 +87,7 @@ install_requires = [
     # * pyOpenSSL >= 0.14 is needed in order to avoid
     #   <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2474>.
     "pyOpenSSL >= 0.14",
+    "PyYAML >= 3.11",
 ]
 
 # Includes some indirect dependencies, but does not include allmydata.
@@ -114,6 +115,7 @@ package_imports = [
     ('six',              'six'),
     ('enum34',           'enum'),
     ('pycparser',        'pycparser'),
+    ('PyYAML',           'yaml'),
 ]
 
 # Dependencies for which we don't know how to get a version number at run-time.