integrate btdigg search api

details on frostwire-desktop repo ([PR #376](https://github.com/frostwire/frostwire-desktop/pull/376))
this section is not my code actually - it was copied from a sibling class 

i have begun documenting and drying out the *SearchPerformer and *SearchResult subclasses and these constants are among several items ive identified that are duplicated verbatim in several sibling classes - this was planned to be the topic of a separate branch

these constants (and the method that uses them) are actually not needed in this class and have since been removed - but they are on my list of items to be factored out into one of their superclass - this class in my working tree currently consists of (besides a constructor) only the data members and their getters - which are all interface common to nearly all sibling classes so after refactoring this particular class will consist of only the constructor