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/main/nim/strawbosspkg/server.nim

@@ -1,4 +1,4 @@
-import algorithm, asyncdispatch, bcrypt, jester, json, jwt, os, osproc,
+import algorithm, asyncdispatch, bcrypt, jester, json, jwt, logging, os, osproc,
   sequtils, strutils, tempfile, times, unittest
 
 import ./configuration, ./core, private/util