Support built-in function authentication with OpenFaaS IAM

Changelog for v0.2.11:

• PR #29 Support built-in function authentication with OpenFaaS IAM. by @welteki

Commits
091053e Accept a http.Request as argument to InvokeFunction by @welteki
7d2f486 Support garbage collection of expired tokens from cache by @welteki
0280ee7 Support configuring the http client for token exchange requests by @welteki
78a05ff Make function token cache optional in client by @welteki
1c4a774 Dump exchange request when FAAS_DEBUG is set by @welteki
e046523 Improve code comments and update README by @welteki
c963f99 Cache function invocation tokens by @welteki
2c63f45 Add support for invoking functions using the client by @welteki
f28717c Improve errors for token exchange functions by @welteki
df1f645 Have TokenAuth type implement the TokenSource interface by @welteki
6099584 Add exchange options by @welteki
cffa985 Add scope to token by @welteki
0f5bb5f Refactor ClientCredentialsTokenSource to use Token struct by @welteki

Changes: v0.2.10...v0.2.11

Generated by Derek