Udocker

* Support for import Docker containers in newer metadata structure
* Improve the command line parsing
* Improve temporary file handling and removal
* Support for additional execution engines to be provided in the future
* Improved parsing of entrypoint and cmd metadata: closes 53
* Increase name alias length: closes 52
* Add support for change dir into volume directories: closes 51
* Fix deletion of files upon container import: closes 50
* Fix exporting of host environment variables to the containers: closes 48
* Change misleading behavior of import tarball from move to copy: closes 44
* Fix validation of volumes specification: closes 43

* Improve download on repositories that fail authentication on /v2
* Improve run verification of binaries with recursive symbolic links
* Improve accelerated seccomp on kernels >= 4.8.0 : closes 40

* Minor bugfixes
* Executable name changed from udocker.py to udocker
* Added support for login into docker repositories
* Added support for private repositories
* Added support for listing of v2 repositories catalog
* Added checksum verification for sha256 layers
* Improved download handling for v1 and v2 repositories
* Improved installation tarball structure
* Insecure flag fixed
* Address seccomp change introduced on kernels >= 4.8.0
* Utilities for packaging
* Improved verbose levels, messaging and output: closes 24, 23
* Fully implement support for registry selection --registry parameter: closes 29
* Provide support for private repositories e.g. gitlab registries: closes 30
* Provide --insecure command line parameter for SSL requests: closes 31

* Initial version