Add ReadOnlyDictionary type

- Add a ReadOnlyDictionary that is similar to the .NET 4.5
  ReadOnlyDictionary
- Add matching tests
- Increment library to version 1.1.0
Update PCL targets

- Updating to the lowest common deominator that VS2013 supports for PCL
  targets
- Since changing the targets is potentially a breaking change,
  incrementing version to 2.0.0
Updated README

- Updated README to fix typos, call out PCL targets, and document
  ReadOnlyDictionary support.