Add tags in db for build

 - 4eb109f is for trac#3014
 - add a N:N relationship for tags/builder.

For the IRCContact PR (I will redo it), we need to get the list of tags related to a build.

Note: with this PR merged to master, we should update the waterfall/console page in order to group builders per tags (like in eight)
I think None is not OK.

There is no semantic difference between None and [], so I think it is better to specify only [] is valid.