added support for findAndModify + javascript integration tests
Add read preference config option.
Fix code review issues: use valueOf(), rename parameter

- Use ReadPreference.valueOf(..) instead of my own implementation
- Rename "readPreference" to "read_preference"
Update readme file.
Merge pull request #49 from kajmagnus79/topic-configurable-read-preferences

Add read preference config option
minor refactoring + updated README.md

included Martijn's code review changes
Merge pull request #47 from day-me-an/findAndModify

added support for findAndModify + javascript integration tests
Fix merge conflicts.