Add public accessors for types to begin supporting XCode 6 beta 4.

Now that XCode 6 beta 4 supports accessors, all of the types that will be externally visible need to be marked public. This is my first stab at this.

Note that Nimble-OSX is still failing to compile with a message of "using bridging headers with framework targets is unsupported". I'm unsure what the problem there is yet.