v.1.5 Add "data" handling.

Allow for configurable (default 1024) bytes of data to be sent to
directly connected clients.
Merge branch 'dev' of github.com:mozilla-services/pushgo into feature/data2

Conflicts:
	src/github.com/mozilla-services/pushgo/simplepush/handlers_test.go
	src/github.com/mozilla-services/pushgo/simplepush/worker.go
r? changes
Fixed maxDataLen default length
r?s

* Added wrappers for maxDataLen to warn or return error
* Moved TFlushReply to worker ad FlushData
Merging #135.
Add handlers config test.
Add balancers.

* Add a `Balancer` interface and `{Etcd, Static, No}Balancer`.
* Enable client redirects. `Worker.Hello()` returns a 302 if a peer is
  available, or a 429 if the cluster is full.
* Clean up etcd-related log messages.
`EtcdLocator`: Don't set `lastFetch` for errors.

Rename the etcd polling and publication functions for clarity.
`loadEnvConfig`: Avoid decoding into nil structs.
Add balancers to the config loading tests.
Bump envconf.