begin attribute allocation at 255

luaotfload will allocate attributes from 127 but never exceed 254.
This patch prevents attribute clashes by offsetting the allocation of
new attributes by 255.
add note to doc about attribute handling
Merge branch 'master' of https://github.com/eroux/luatexbase
merge eroux/master
Merge pull request #2 from phi-gamma/master

pull changes from eroux
Merge pull request #5 from lualatex/master

update with lualatex master