core, memory, bridge: added profile password + password encryption:

/!\ This changeset updates the database version to 2 and modify the database content!

Description:
    - new parameter General / Password to store the profile password
    - profile password is initialized with XMPP password value, it is stored hashed
    - bridge methods asyncCreateProfile/asyncConnect takes a new argument "password" (default = "")
    - bridge method asyncConnect returns a boolean (True = connection already established, False = connection initiated)
    - profile password is checked before initializing the XMPP connection

    - new private individual parameter to store the personal encryption key of each profile
    - personal key is randomly generated and encrypted with the profile password
    - personal key is decrypted after profile authentification and stored in a Sessions instance
    - personal key is used to encrypt/decrypt other passwords when they need to be retrieved/modified

    - modifying the profile password re-encrypt the personal key
    - Memory.setParam now returns a Deferred (the bridge method "setParam" is unchanged)
    - Memory.asyncGetParamA eventually decrypts the password, Memory.getParamA would fail on a password parameter

TODO:
    - if profile authentication is OK but XMPP authentication is KO, prompt the user for another XMPP password
    - fix the method "registerNewAccount" (and move it to a plugin)
    - remove bridge method "connect", sole "asyncConnect" should be used

src/core/constants.py

@@ -40,6 +40,10 @@ class Const(object):
     NO_SECURITY_LIMIT = -1
     INDIVIDUAL = "individual"
     GENERAL = "general"
+    # Parameters related to encryption
+    PROFILE_PASS_PATH = ('General', 'Password')
+    MEMORY_CRYPTO_NAMESPACE = 'crypto'  # for the private persistent binary dict
+    MEMORY_CRYPTO_KEY = 'personal_key'
     # Parameters for static blog pages
     STATIC_BLOG_KEY = "Blog page"
     STATIC_BLOG_PARAM_TITLE = "Title"