v2: adopt begin/end semantics to half-open interval

[andig]

Fix #69.

Warning: breaking change.

This PR changes the begin/end semantics as discussed in https://github.com/jinzhu/now/issues/69. If it should be accepted (chances are probably low), this should come as a v2 release.

PR also contains some reformatting due to gofmt, see https://goreportcard.com/report/github.com/jinzhu/now.