Version 0.3.1

Major changes:
- Implemented container delegation.
- Implemented memory eviction_priority in supported kernels.
- Added FD-related FilesystemStats.
- Added "update" command to the CLI.
- Some plumbing for namespace support.
- Misc refactorings, bugfixes, and improvements.
Updating version number in Makefile
Fix building with gcc-4.7.2.
Merge pull request #12 from ezhuk/gcc-check

Fix building with gcc-4.7.2.