travis-ci integration (pull-tester replacement)

See the doc/travis.txt for a description of what travis is and how it works.

This is an initial descriptor to get things going. It can be changed around at any time, so discussions about what build configs to use, notifications, etc, can all take place in the form of pull-requests in the future.

At the moment, each pull-request will kick off 7 builds:
- linux x86_64 full
- linux x86_64 minimal
- linux i686 full
- linux arm no-qt
- osx x86_64 full
- win x86_64 full
- win i686 full

Build results will be similar to this: https://travis-ci.org/coryfields/bitcoin/builds/33137850 .

There are lots of features and bells/whistles to play with down the road, see [notifications](http://docs.travis-ci.com/user/notifications/) for an example.

There's also a quick build change in here to make osx build logs actually useful.
@sipa: These are non-debug, so significantly smaller than typical dev objects. They're also gzip compressed (see 2 lines down). A full linux build that way results in a ~17mb ccache dir. For reference, the exact same build _with_ debug is 105mb. That's probably more inline with what you're seeing?

Also, as I see it, there's really no use in keeping more than one version of each object around, since each cache matches a single build config. It either matches the current revision or it can be discarded. So my target with the 100mb figure was the largest reasonable compressed store of one version of all objects.