Python unicode decode errors when decrypting.

See merge request matrix-org/olm!4

This is needed because the function returns Unicode strings and the
comparison will fail under python2 unless Unicode literals are used.

explicitly define backward & forward secrecy

See merge request matrix-org/olm!3

The to_native_str function was supposed to produce Unicode decoded
native strings for python2 and python3.

Upon further consideration this doesn't make much sense since under
python2 it would need to decode the bytes into a Unicode string and turn
it back into a python2 str.

The ability to use the replacement character requires us to use a
Unicode string under python2 as well.

This patch changes the decryption functions not to fail if there was an
unicode decode error while converting the decrypted bytes plaintext into
a native python string.

Characters that cannot be decoded as unicode are now replaced with the
unicode replacement character (U+FFFD).

The old behaviour of raising an UnicodeDecodeError can be achieved by
passing the "strict" error handling scheme to the decrypt function.

as it repeatedly trips people up, including me

Convert docs from RST to Markdown

See merge request matrix-org/olm!2

as per https://github.com/matrix-org/olm/issues/15.
thanks to @dest

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

also make some params const where possible

cmake: Add the SAS functions to the CMake build.

See merge request matrix-org/olm!1

Signed-off-by: Damir Jelić <poljar@termina.org.uk>