Merge pull request Haivision#333 from ethouris/maint-mergeback-dev

Dev branch mergeback

Bump version to 1.2.3

Bump up SRT_VERSION to 1.2.2

This patch is a fix for last released version.
Although 1.2.1 has been release, soname and version string in
.pc file still stay 1.2.0.
To avoid confusion, one minor version is bumped up.

Signed-off-by: Justin Kim <[email protected]>

Add support nettle AES cryptographic. (Haivision#149)

SRT supports nettle AES encryption using
`--with-gnutls` option at build time.

Signed-off-by: Justin Kim <[email protected]>

Merge pull request Haivision#75 from ethouris/extra-doc

Added some extra info to contributing guide