Capstan v0.3.0

• add capstan package update to update all local packages
• some minor bugfixes
• sequential config_set support
• allow remote unikernel contextualization (compose-remote)
• volumes support (create, attach)
• ensure compatibility with Mac by using safer QEMU flags
• add 3-clause BSD license
• unikernel images now store creation time and platform
• capstan pacakges now store creation time and platform
• add Python runtime
• bootcmd inheritance from parent packages
• support --env also in capstan package compose
• recursive boot commands
• package self-documentation
• compressed capstan packages