Replace pthread with boost::thread
Add boost thread in the travis install script
The return value of WaitForInternalThreadToExit has reversed
Merge pull request #1000 from kloudkl/boost-thread

Replace pthread with boost::thread
Create base data layer and base prefetching data layer
Extract common data layer functionalities out of the DataLayer
Remove duplicate codes from the ImageDataLayer
Simplify the WindowDataLayer using the base class
The BasePrefetchingDataLayer shouldn't join the thread
Implement Forward_gpu in the base prefetching data layer
Add utility function to convert OpenCV image into Datum and Blob
Add transformer to the memory data layer