Use mox for writing mocked tests of Merge

With @sbesson's changes, it now should be
fairly straight-forward to write mocked
tests. This is currently done using mox.
Like Sandbox.py, Mocks.py provides a base
class which should be used by all mocked
tests.
Add missing test/Mock.py
Pass filters directly to run_filter() and add action optional argument
Add filtering unit tests
Remove get_user() get_organization() function overloading

These overloaded functions (for mox testins) do break the instantiation of
real repositories.
Replace lstrip by replace to fix bug

Example of filter that was not working properly with lstrip(): "user:sbesson".
In this case, s.lstrip(key + ":") will also strip the first "s" character.
Add default optional argument to control the whitelisting mode

Define 4 default modes: none, mine, org and alls. Allow better granularity and
potentially any possibility when combined with include/exclude filters.