on debug does not minify html

If settings.DEBUG is True developer does not want minifyed html. This
is needed only in production enviroment.

avoid errors for newlines in stderr

UglifyJS2 prints two newlines '\n\n' into stderr for me.
The `if stderr` in the `SubProcessCompressor.execute_command` raises a CompressionError if that happens.
I've added a `strip()` call to avoid that.

re-add proper Jinja2 support

Conflicts:
	pipeline/jinja2/ext.py

skip non-existing files in bundles

Fixed SyntaxError syntax raised by TemplateSyntaxError.

If there are non-existing source files in bundle definitions, we have
to skip those. This can be very useful for large projects with many
apps (optional apps) but one `settings.py`.