React to new Roslyn API
Disable concurrent build on mono because of a PLinq bug
Refactored NuGetDependencyResolver to be more robust
- Changed 5 dictionaries into 2
- Made a single structure that represents all package information
  to be used for subsequent calls
- Properly map assemblies names to paths for loading
- Use all reference assemblies in the package for compilation

#266
Make commands in project.json case-insensitive
Refactor command-line parser

- Add CommandOptionType in CommandLineApplication parser
- One option can have multiple values in CommandLineApplication now
- Add helper methods that generate help information in
CommandLineApplication
- Replace all usages of CommandLineParser with CommandLineApplication
parser
- Merge CommandLineApplication and CommandInfo into one class to allow a
more consistent processing
- Help options are treated specially because they are different from
common options (help options invoke actions)
- Help command is added automatically when an app has some commands
- Show short hint rather than full help if find invalid options or args
- Add symbol option to merge all three help options
Cleaned up command line parsing
- Removed /arg support because it breaks non windows platforms
- k by itself doesn't work anymore, you must use k run