Added Circle CI integration to run Android unit tests

Summary:
A few caveats before accepting:
- Do I need to squash commits?
- Need to set up new Circle CI account connected to FB react-native repo
- After that replace tokens and links to the new ones

Setting up Integration tests should be straight forward next week https://circleci.com/docs/android
Closes https://github.com/facebook/react-native/pull/4566

Reviewed By: svcscm

Differential Revision: D2723119

Pulled By: androidtrunkagent

fb-gh-sync-id: 2c4a46b206f15f36d94d1b10ff9e339f5182782b

circle.yml

@@ -0,0 +1,12 @@
+machine:
+  node:
+    version: 5.1.0
+
+dependencies:
+  pre:
+  - npm install -g npm@3.2
+
+test:
+  override:
+    # gradle is flaky in CI envs, found a solution here http://stackoverflow.com/questions/28409608/gradle-assembledebug-and-predexdebug-fail-with-circleci
+    - TERM=dumb ./gradlew cleanTest test -PpreDexEnable=false -Pcom.android.build.threadPoolSize=1 -Dorg.gradle.parallel=false -Dorg.gradle.jvmargs="-Xms512m -Xmx512m" -Dorg.gradle.daemon=false