upgrade to auth0.v2 and fix email templates

[alexkappa]

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Fixes #104

Changes proposed in this pull request:

• Upgrade to gopkg.in/auth0.v2
• Fix resource/auth0_email_template 404 issue
• Make any change to resource/auth0_custom_domain force a new resource. Auth0 Management API no longer supports update (HTTP PUT) and Auth0 SDK followed suit.

Output from acceptance testing:

$ make testacc TESTS=TestAccEmailTemplate                [17:26:03]
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./...) -v  -timeout 120m -coverprofile="c.out" -run ^TestAccEmailTemplate
?       github.com/alexkappa/terraform-provider-auth0   [no test files]
=== RUN   TestAccEmailTemplate
--- PASS: TestAccEmailTemplate (1.12s)
PASS
coverage: 18.7% of statements
ok      github.com/alexkappa/terraform-provider-auth0/auth0     1.165s  coverage: 18.7% of statements