Added support for request headers. :rocket:

Allows for either hardcoded or dynamic request headers, using a similar
approach to retrofit.
Set content headers and allow header removal/redefinition
Fix 2 of the 3 broken integration tests.

The two GitHub API tests were failing because the API requires a User-Agent header.

NFI what the "PostToRequestBin" test is supposed to do, so I left it broken.
Adds request header info to README
Style stuff I overlooked before
Merge remote-tracking branch 'origin/pr/31'