Add authentication framework

[jeremyrsmith]

• Add the ability to place filters before the HTTP service, configurable at the request level
• Add the concept of an Authorizer, which is just a filter.
• Add some initial Authorizers - Bearer (for pre-shared Bearer tokens) and MAC (for pre-shared MAC tokens)

A future PR should contain a full OAuth2 flow implemented in an Authorizer.

[codecov-io]

Current coverage is 54.48% (diff: 20.23%)

Merging #44 into master will decrease coverage by 8.87%

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- Misses           96        142    +46   
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[ilya-murzinov]

@jeremyrsmith is it possible to write a test for Bearer and MAC authorizers?