Migrate to Gradle (#1159)

* Start gradle root

* Clean up test API ambiguities

These don't compile in gradle anymore and the team[] warns

* Exclude .gradle dirs in git

* Set up moshi module

* Set up moshi-adapters

* Add repositories to subprojects

* Set target/source compatibility

* Set up examples

* Fix location of reflect/test dir

* Set up moshi-kotlin

* Set up code gen

* Opportunistic update code gen deps

* Fix up with code gen

* Set up kotlin tests

* Update snapshots

* Update travis build

* Configure checkstyle

* Cache gradle

* Finish fixing up checkstyle

* Now disable checkstyle until we can fix them all :|

* Update contributing

* Fix tests in codegen

* Remove unnecessary annotation

* Remove maven stuff!

* Suppress warning

* Remove jcenter

* Consolidate dependencies

* Revert "Clean up test API ambiguities"

This reverts commit 3ead69b844b5d7f66134b721e95581f5df1cccd6.

* Fix incap dep

* Opportunistically fix some small kotlinpoet deprecations

* Automatically apply the stdlib to all kotlin projects

* Opportunistic move to opt-in and remove unnecessary annotations

The kotlin maven plugin didn't handle these well in the IDE, gradle does

* Fix Type doc warning

* Fix okio version

* Fix dokka support

* Fix copypasta

* Use new snapshot

* Kotlin 1.4.0

kotlin/reflect/src/assembly/dokka.xml

@@ -1,16 +0,0 @@
-<assembly
-    xmlns="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.3"
-    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:schemaLocation="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.3 http://maven.apache.org/xsd/assembly-1.1.3.xsd">
-  <id>javadoc</id>
-  <formats>
-    <format>jar</format>
-  </formats>
-  <baseDirectory>/</baseDirectory>
-  <fileSets>
-    <fileSet>
-      <directory>target/dokka/moshi-kotlin</directory>
-      <outputDirectory>/</outputDirectory>
-    </fileSet>
-  </fileSets>
-</assembly>

kotlin/reflect/src/test/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt

@@ -9,7 +9,6 @@ class KotlinJsonAdapterTest {
   @JsonClass(generateAdapter = true)
   class Data
 
-  @ExperimentalStdlibApi
   @Test
   fun fallsBackToReflectiveAdapterWithoutCodegen() {
     val moshi = Moshi.Builder()