v0.1.8

Feature: Get rid of the dependency to external protobuf
Feature: added (Encode|Decode)AsSerializedProto interface so Python module can get full access to the SentencePieceText proto including the byte offsets/aligments
Feature: added --treat_whitespace_as_suffix option to make _ be a suffix of word.
Feature: Added normalization rules to remove control characters in the default nmt_* normalizers
Minor fix: simplify the error messager
Minor fix: do not emit full source path in LOG(INFO)

For more detail: v0.1.7...v0.1.8