Road to v2.0

Based on the changes by @patbenatar I started iterating towards v2.0, this is the current WIP. Personally, I think it got much more cleaner, more readable, more extensible and generates fewer methods. Due to this some parts will be not be backwards compatible.

- [x] <del>Make validations work</del> makes apart from `:presence` no sense
- [x] Support `:whiny` again
- [x] Support transforms like `strings: true` did previously, still required/useable?
- [x] Clean up specs, remove obsolete specs etc.
- [x] Add mongoid support, should be trivial in fact
- [ ] Finish new README
- [ ] Update gh-pages... maybe find a designer for a neat logo
- [ ] jruby/mongoid issues: `Bundler could not find compatible versions for gem "bson":
    mongo (~> 1.9.0) java depends on
      bson (~> 1.9.0) java
    mongoid (= 4.0.0.beta1) java depends on
      bson (2.0.0.rc2)`

/cc @the-robear