Stabilization of v0.1.57

[Vojtech Polasek]

Hello all,
*Release of Content version 0.1.57 is scheduled for July 23rd*, so I've 
created a stabilization branch for it.
Now it is a good time to hunt for bugs and propose fixes for them. 
Starting today, merged Pull Requests should be assigned to milestone 0.1.58.
Bug fixes should be proposed to master and labeled "bugfix", they will 
be cherry-picked to the stabilization branch.
The next release 0.1.58 will happen on September  24th. Stabilization 
phase will start on September 13th.
Best regards,