documentation and small changes

- added docs for `Promise` and  `PromiseFactory`
- removed outdated README docs
- moved some common utilities to `PromiseUtil`
- improved efficiency of result array resizing
- added cancellation result to promise executors
- changed visibility of `PromiseJoiner` to public, and made some method names more verbose
- inlined `DeferredExecutionException` to inside `AbstractPromise`
- inlined default promise implementation to inner class in the factory
- removed necessity for base factories to provide a logger

build.gradle

@@ -14,7 +14,7 @@ nexusPublishing {
 
 subprojects {
     group = 'dev.tommyjs'
-    version = '2.4'
+    version = '2.4.0'
 
     apply plugin: 'java-library'
     apply plugin: 'com.github.johnrengelman.shadow'
@@ -43,6 +43,7 @@ subprojects {
         useJUnitPlatform()
         testLogging {
             exceptionFormat = 'full'
+            showStandardStreams = true
         }
     }
 }