remove app engine code and references

[kellymason]

Goal

Removes support for the old version of Google App Engine that Google has now dropped.

Design

All references to google app engine and related code should be removed.

Removed

• appengine.go
• appengine-test.go
• app engine example app
• documentation references

Tests

I ran go test and received the following issues, but if I understand correctly this is a known issue:

    bugsnag_test.go:627: Expected frame file to be 'runtime/panic.go' but was 'runtime/chan.go'
    bugsnag_test.go:630: Expected frame method to be 'gopanic' but was 'chansend'
    bugsnag_test.go:627: Expected frame file to be 'bugsnag_test.go' but was 'net/http/server.go'
    bugsnag_test.go:630: Expected frame method to be 'crashyHandler' but was 'HandlerFunc.ServeHTTP'
    bugsnag_test.go:638: Expected frame inProject to be 'true' but was 'false'

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • [ x ] Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[kinbiko]

@kellymason The test failures you saw are new in Go 1.12 (latest version), and have been fixed in #105. Presumably, these only appear locally.

[kinbiko]

Looks good to me!