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

gradle.properties

@@ -0,0 +1,31 @@
+#
+# Copyright (C) 2020 Square, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+kapt.includeCompileClasspath=false
+
+GROUP=com.squareup.moshi
+VERSION_NAME=1.11.0-SNAPSHOT
+POM_DESCRIPTION=A modern JSON API for Android and Java
+POM_URL=https://github.com/square/moshi/
+POM_SCM_URL=https://github.com/square/moshi/
+POM_SCM_CONNECTION=scm:git:git://github.com/square/moshi.git
+POM_SCM_DEV_CONNECTION=scm:git:ssh://git@github.com/square/moshi.git
+POM_LICENCE_NAME=The Apache Software License, Version 2.0
+POM_LICENCE_URL=https://www.apache.org/licenses/LICENSE-2.0.txt
+POM_LICENCE_DIST=repo
+POM_DEVELOPER_ID=square
+POM_DEVELOPER_NAME=Square, Inc.
+POM_DEVELOPER_URL=https://github.com/square/