Section 10: Clarify language around build metadata precedence

They're not the 'same version', necessarily, they're the same precedence
Clarify non-empty build metadata

- Build metadata identifiers MUST not be empty.
- Changed "are" to "have" when discussing version precedence. We usually
say a version HAS a precedence not that it IS a precedence.
Be more precise about version precedence
Merge pull request #109 from mojombo/build-metadata-clarification

Build metadata clarification