goken17: Quick and dirty power-assert implementation in golang.

Quick and dirty power-assert implementation in golang.

```
$ GOPATH=`pwd` go test src/twada/powerasserttest/powerassert_test.go
--- FAIL: TestNotPowered (0.00 seconds)
	normal.go:9: failed
--- FAIL: TestEmpoweredAssert (0.00 seconds)
	powered.go:37: Assertion Failed.
		ASSERT:
		   powerassert.Ok(t, hoge == fuga)
		WHERE:
		   hoge => foo
		   fuga => bar
--- FAIL: TestEmpoweredAssertWithLiteral (0.00 seconds)
	powered.go:37: Assertion Failed.
		ASSERT:
		   powerassert.Ok(t, piyo == "fuga")
		WHERE:
		   piyo => toto
FAIL
FAIL	command-line-arguments	0.026s
```