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 can remove it. But, to me, it's better/safer to keep this kind of check. 
The configuration module set tags to None by default. If I remove this line, I will patch the config module to set tags to [] by default.