buildbot: broke into a master and a builder bot

This change is the first major step towards more improvements to the
buildbot. It cleans up various details, but most notably:
-it now uses a task as the basic data structure, to improve readability
and maintainability
-it breaks the bot into a master bot and a builder bot.
-it allows a master to receive reports from remote bots, and the
status page now aggregates all those reports.
-it allows a builder bot to send its reports to several master bots.
-the master bot uses whatever go version is installed to build the
builder bot, or the downloaded go tip if none is found.

The master bot is in charge of
1) monitoring the changes to the Camlistore and Go repos,
2) recompiling builder bot, which allows to add some new tests to the
test suite without having to interrupt the master bot.
3) starting a builder bot instance
4) waiting for the builder bot report and/or polling the builder bot
for its progress
5) answering the requests to display the various status pages.

http://camlistore.org/issue/185

Change-Id: I46a0b8fabbebf76b0c3ed04fd2ee73362d565cf7