Debian package: Add support for xulrunner 28

Also bump upstream version in changelog.
.deb nightlybuild script: No more warning about using --ignore=wrongdistribution
.deb nightlybuild script: Use git gc to keep .git reasonably small
Update coroutines to support Promises, cancelation

`co_call' and SUSPEND/CONTINUATION are now deprecated, and `spawn'
should be used instead for Promise and cancelation support.

modules/compat/Promise.jsm is used for Gecko versions older than 25.
It is derived from the Firefox 26.0 Promise.jsm, but was changed to be
syntax-compatible with Firefox 4.0.
Replace uses of co_call/continuation API with uses of spawn/Promise API
buffer.js: Fix this.container.selectedPanel is undefined error

This occurs with Gecko >= 25
Support Gecko >= 26 Downloads.jsm interface
buffer.js: Properly fix lack of current buffer after removing a buffer

buffer_container now has a current_buffer_object that tracks the
current buffer, as the selectedPanel attribute of the xul:deck cannot
be relied upon while removing a buffer.