DRY up the use of fromHex and put it in ethutil

ethutil/common.go

ethutil/common_test.go

rpc/api.go

rpc/util.go

rpc/util_test.go

ui/qt/qwhisper/whisper.go

xeth/state.go

xeth/types.go

xeth/whisper.go

xeth/xeth.go