added and modified database from DBow3

[tmako123]

I added and modified database from DBow3.
Check fbow_demo with argument
"orb ../../utils/images/image0.png ../../utils/images/image1.png ../../utils/images/image2.png ../../utils/images/image3.png"

[rmsalinas]

Hi, did you have the time to test the changes and verify it works? I'm planning to integrate them into the trunk

[ake020675]

hi ,i tested your changes and found errors caused by "export.h" in "Database.h" and "QueryResults.h". I changed it to be "flow_export.h" and all_build succeeded.

[ake020675]

Found other problems in "fbow_demo.cpp":
1 fbow::voc.save only support binary fromat;
2 database.save supprot "txt" or "yml" format because it runs with cv:Filestorage.
So i changed voc.saveToFile("small_voc.yml.gz") to voc.saveToFile("small_voc.bin"), and
db.save("small_db.yml.gz") to db.save("small_db.yml").
Now everything seems correct.