initial work on refactoring mongosplitter code.
incremental work on mongo splitter
add specific exception class for cases where splits can't be calculated successfully
finish up class-based reimplementation of splitting
hook new splitting implementation into inputformat APIs.
make mongosplitter an abstract class, move splitter implementations into subclasses
add test for multiple mongos outputs
Merge branch 'master' into splitter-refactor
remove code folding from mongoinputsplit
hadoop version in build.sbt should be left as default.
don't expose implementation of dbobject in config util.
splitter refactor tweaks from codereview.