Split testing into unit and functional tests.

* Split the `test` nimble task into `unittest` and `functest`, with
  corresponding test directories and test runners.
* Added documentation in README regarding building and testing StrawBoss.
* Created a small, simple test project for use in the functional tests.
* Added a `keepEnv` template in the server unit test code to make it easy to
  preserve the working environment for a single unit test to invistigate
  failures manually.

src/test/nim/testutil.nim

@@ -0,0 +1,9 @@
+import httpclient, json, strutils
+
+proc newAuthenticatedHttpClient*(apiBase, uname, pwd: string): HttpClient =
+  result = newHttpClient()
+  let authResp = result.post(apiBase & "/auth-token", $(%*{"username": uname, "password": pwd}))
+  assert authResp.status.startsWith("200")
+  result.headers = newHttpHeaders({"Authorization": "Bearer " & parseJson(authResp.body).getStr})
+
+