Merge pull request #13 from phi-gamma/master

make unset value accessible by a function
Merge pull request #15 from phi-gamma/master

update with requests by @sh2d
Merge pull request #8 from lualatex/master

sync with lualatex repo
It can be normal for a function to return false
In simple callbacks, we stop on a false return

(so that we can truely disable hyphenation)
Adding a test
Bumping date and refreshing callback list
updating NEWS file
Logging is just spoiling logs when done with hyphenate callback...
Merge pull request #16 from eroux/master

Minor improvements in mcb, going 0.7
[mcb] implement is_active_callback()

Part of https://github.com/lualatex/luatexbase/issues/17
[attr] fix error message
[mcb] add tests for is_active_callback()
[mcb] implement callback_descriptions()

Addresses https://github.com/lualatex/luatexbase/issues/17
[mcb] add test for callback_descriptions()
update news