eventer: fix tests
eventer: add test for concurrent Post/Register

This test reports the race condition when run
using "go test -race".
eventer: fix data race
eth: fix filter map data race

This commit also documents (but doesn't enforce) that filters
are immutable while they're installed. This required a minor API
change.
Implemented new iterator
Merge pull request #56 from fjl/feature/raceless-eventer

Fix Eventer race
Merge pull request #57 from fjl/feature/raceless-ethereum-filters

Fix filter map race
Merge branch 'develop' of github.com-obscure:ethereum/eth-go into develop
Added generic big to 256 method. Implemented new iterator
Added some methods to comply to the PoW block interface
Implemented AR PoW
Max callstack
event: new package for event multiplexer
all: use (blocking) event package instead of ethreact
ethchain: fix tests