Added support for discovering injectable ("Burstable") fields.

Implements square/burst#36.
We are actually only finding public constructors currently – both [JUnit4](https://github.com/square/burst/blob/ea89bbeaa99729bc6925bc066b78f48e9bb2f4d0/burst-junit4/src/main/java/com/squareup/burst/BurstJUnit4.java#L80) and [Android](https://github.com/square/burst/blob/ea89bbeaa99729bc6925bc066b78f48e9bb2f4d0/burst-android/src/main/java/com/squareup/burst/BurstAndroid.java#L135) use Class.getConstructors(), which, according to the javadoc, only returns public constructors. There are even unit tests (on [Android](https://github.com/square/burst/blob/ea89bbeaa99729bc6925bc066b78f48e9bb2f4d0/burst-android/src/test/java/com/squareup/burst/BurstAndroidTest.java#L54) and [JUnit4]()) for this behavior.