Simple thresholding

For some issues Otsu thresholding might not be useful. For example #78 (this PR could fix it).
I've implemented method to convert UIImage to Pix(implementation from [thresholder.cpp](https://code.google.com/p/tesseract-ocr/source/browse/ccmain/thresholder.cpp?name=3.03-rc1#54) was used) and custom image setter with simple thresholding.

Also I've found that Tesseract creates inner copy of Pix so it could be released after setting it up. I've create a [ticket](https://code.google.com/p/tesseract-ocr/issues/detail?id=1396&can=6&sort=-id) with question for API documentation changes. 

For #85 there were some problems with ```let``` specs. I've rewrited them with ```__block``` variables. Looks more clear now. New functionality was covered with tests also.