Only export public symbols.
Merge pull request #414 from sparkle-project/visibility

Only export public symbols.
Fixed Sparkle tags parsing
Merge branch 'tamasprezi-tags_fix'
Prefer string constants to strings.
Rename Sparkle appcast keys to make clear elements vs attributes.
Fix minor spelling error.
Use string constants for RSS element and attribute names.
Eliminate a few more string constants.
Use string constants in system profiler.
Add ARM support to the system profiler as OS X might later run on it.
Avoid hardcoded file paths.
Share the distribution scheme and configure it to run tests.

This way, we don't rely on automatic scheme creation in Travis, nor do
we have to update the Makefile for Travis if we want to add an
additional test target to the project.

Also, Travis should now build all targets (including BinaryDelta and
Sparkle Test App), giving us better build coverage.
Fix package building for Debug configuration.
Add instructions to the README for building release packages.