support nested txns in ShadowSQLiteDatabase
Merge pull request #94 from loganj/loganj/txn_stack

support nested txns in ShadowSQLiteDatabase
Merge remote-tracking branch 'square/master' into robolectric-2-master

* square/master:
  support nested txns in ShadowSQLiteDatabase

Conflicts:
	src/main/java/com/xtremelabs/robolectric/shadows/ShadowSQLiteDatabase.java
	src/test/java/com/xtremelabs/robolectric/shadows/DatabaseTestBase.java