Bulker

- Better error message if user tries to activate a crate with *no* crates loaded.
- Uses default shell by default, and can be specified with the `shell_path` in config.
- Singularity will now automatically pull new images without requiring `-b` on load
- Added new attributes to disable user or network maps (`no_user` and `no_network`).
- Revamped the templates to fix a few small convenience issues
- Switch to using symlinks instead of wrapper scripts for host executables
- require latest yacman

- Renamed 'dockerargs' to 'docker_args' and 'singularityargs' to
'singularity_args', which is more consistent with other attribute styles.
- Implemented prototype container entry system, whereby you may type `_command`
after activating a crate to enter an interactive shell of the container that
is used to run that command.
- Initial implementation of 'imports' domain capability.
- Added the `tool_args` bulker config section for host-specific *and*
image-specific settings

- Fixed a bug with file locks that prevented activating environments
- Relative paths for singularity images in the bulker config are now made
relative to the config file, rather than to the working directory.

- Upgraded yacman to protect against two bulker processes writing to the config
file at the same time.

- Fixed a bug with using the built-in config file

- Fix problem with python2 compatibility
- Init now copies over templates, which are relative to the config file. This
makes it easier to share a bulker configuration in a shared computing
environment.
- update registry to *hub.bulker.io*.