Update changelog and bump version to v0.91-beta1 (911)

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Bump toolchain to NDK r10d

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Bump stoken to 0.8-rc

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Use gmp/nettle instead of tommath/tomcrypt

The former libraries are a little easier to work with because nettle
has a proper pkg-config file.  Also, libtom.org is now offline.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Bump libxml2 to 2.9.2

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>