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)
The hashes are to work around MySQL's inability to use arbitrary-length strings in its unique columns.  I other words, we can't use `sa.Column('name', sa.Text, unique=True)` because Text is too wide for MySQL to handle.