Signedimp

* Add ability to unsign a previously-signed frozen app, but inserting
a DummySignedImportManager into the bootstrapping code.
* Fixes for rpython esky compat script:
* compile under PyPy 1.4
* don't try to use O_BINARY on non-windows platforms

* Removed separate hashing of modules vs data files - all files are now
hashed independently and the various files that make up a module import
are checked independently at runtime.
* Conform to some subtler points of the PEP302 importer spec, with respect
to module reloading and sys.meta_path.
* Add RPython compatability to signedimp.compat.esky
* It is no longer possible to specify 'randbytes' at the module level
in the various cryptobase modules, you must explicitly pass a callable
to each constructor.

* Factor padding algorithm out from the RSA key class, so that it's easier
to upgrade to a new scheme:
* RSAKeyWithPSS is no more, just use RSAKey
* the signature produced by RSAKey.sign() is prefixed with the name
of the padding scheme used.
* "pss-sha1" is the default padding scheme; change it by setting
the attribute "default_padding_scheme" on the key.
* output hash data in sorted order, for easier debugging by hand.
* moved signedimp.pkgres to signedimp.compat.pkgres.
* added signedimp.compat.esky, providing code to add signedimp support when
freezing an application with esky.
* tools.get_bootstrap_code() now returns code fully equivalent to doing
"import signedimp", i.e. it initialises the signedimp module and assigns
it to the local name "signedimp".

* better support for namespace packages and other abusers of __path__.
* better support for frozen packages.
* tools.sign_zipfile: don't modify files that are not already zipfiles.
* ability to record "module aliases" for cases where modules can be
loaded under something other than their canonical names (Django template
tag libraries, I'm looking in your direction...)

* implement basic integration with pkg_resources - just import the
"signedimp.pkgres" module to install the necessary hooks.
* have custom bootstrap code create an actual "signedimp" module, so
it's easy to tell that it has been installed.

* use fast RSA primitives when PyCrypto is installed.
* do some simple caching to speed up imports from the same source.
* replace imp.load_[dynamic|compiled|source] with custom wrappers.
* necessary for good interop with py2exe and py2app, which use
imp.load_dynamic to load DLLs from the filesystem.
* when bootstrapping a frozen executable, check whether signedimp is
already present and shortcut if so.
* necessary for good interop with esky on win32, where it tries to
chainload into the frozen executable from an existing process.