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

.buildscript/deploy_snapshot.sh

@@ -21,6 +21,6 @@ elif [ "$TRAVIS_BRANCH" != "$BRANCH" ]; then
   echo "Skipping snapshot deployment: wrong branch. Expected '$BRANCH' but was '$TRAVIS_BRANCH'."
 else
   echo "Deploying snapshot..."
-  ./mvnw clean source:jar javadoc:jar deploy --settings=".buildscript/settings.xml" -Dmaven.test.skip=true
+  ./gradlew uploadArchives
   echo "Snapshot deployed!"
 fi