Add: set initialPair by images name
Update main_IncrementalSfM.cpp

Fix typo
Merge branch 'dev_namedInitialPair' of https://github.com/mikrosimage/openMVG into mikrosimage-dev_namedInitialPair
Repair initial behavior pair parameters (-a, -b are optional parameter) #200.
Merge branch 'mikrosimage-dev_namedInitialPair' into develop