Add support for the auto_build setting which was introduced in webassets 0.7

Webassets introduced the "auto_build" option in 0.7 and deprecated setting updater=never or updater=False . This patch adds the auto_build option as a boolean.

Note:
Many of the tests seem to be machine-dependent (trying to write to '/home/sontek' for example). This could probably be improved, along with improved documentation on how to run the tests.