Fixes #66 by adding Writer::Reset() and multiple root check

Note it redefines RAPIDJSON_ASSERT() to throw exception in unittest and
check for assertion with gtest.
Add Writer::IsComplete()
Add API doc for Writer::Reset() and Writer::IsComplete()
Merge pull request #67 from miloyip/issue66writerassertion

Fix #66 by adding Writer::Reset() and Writer::IsComplete()