Upgrade to Go Version 1.7

So far spiff used a very old Go version. On MacOS version Sierra the Go runtime does not work correctly anymore. This problem is solved with the latest Go version 1.7. Therefore spiff has been upgraded to this Go version.

Additionally a new function merge(map1,map2) has been added. This function allows merging in-document map nodes with the same semantics so far only available for the multi-document merging feature of spiff.