Fix plenty of clang warnings declaration shadows a variable: using class enum instead of enum

[hyperxor]

I have got a lot warnings from clang compiler:

yaml-cpp/src/emitterstate.h:28:24: warning: declaration shadows a variable in namespace 'YAML' [-Wshadow]
  enum value { NoType, Flow, Block };
                       ^

and my proposal is to use enum classes.

[jbeder]

This is probably right, but it's a breaking change (e.g. see https://stackoverflow.com/questions/10361507/link-compatibility-of-enums-and-enum-classes), so we'll bump the version to 0.7.0 next.

Can you make sure you've run clang-format on the changed lines?

[hyperxor]

@jbeder Hello, unfortunatelly, this change breaks API, and shouldn't be done in minor version.
I run clang-format and push the following changes. In general, I think this change makes API clear and better, but I'm not hurry and it can be merged to master later.

[rickyjames35]

I see this got closed by never made it into a release. Any chance we can get this in?