Our magic value wasn't being used and were represented as a string.
Refactored it to a constant, increased it's size to 2 bytes and optimzed
is_symmetrically_encrypted to look for the magic and symmetrically
encrypted flag under base64 encoding. Most file types will use this
feature to help identifying themselves, so it got refactored to serve
the purpose it was created.

Those are already present, but we are using the ones coming from our
dependencies. Explicit is better than implicit.

modifying original PR [0] by cristoph to account for the recent
vendoring of l2db code, which means we no longer depend on u1db/dirspec.

I expect the whole mess about the venv setup to be further simplified
pretty soon, since we are going to merge most of the leap.* packages
into a couple of repos.

[0] https://github.com/leapcode/soledad/pull/327

From this moment on, we embed a fork of u1db called l2db.

Old versions of pip do not accept the --trusted-host option and will complain
when trying to upgrade pip from wheel. To fix that we upgrade pip from usual
location instead of doing it from wheel.