Port toml-test to pure Go (#264)

* Port toml-test to pure Go

This change basically ports the toml-test examples test suite to pure
Go. This removes the snowflake test.sh required to run such tests, and
allows us to the example tests on any platform (which includes Windows
as part of the pull-request testing).

* Allow CircleCI failure for go tip

CONTRIBUTING.md

@@ -83,9 +83,7 @@ some early feedback!
 
 #### Run the tests
 
-go-toml uses two kind of tests: unit tests and TOML example tests.
-You can run both of them using ./test.sh.
-
+You can run tests for go-toml using Go's test tool: `go test ./...`.
 When creating a pull requests, all tests will be ran on Linux on a few Go
 versions (Travis CI), and on Windows using the latest Go version
 (AppVeyor).