Closes #269

Closes #254.

Specification refactor

Support for packaging storyboards in pods

- Usually only the header files are commented.
- The header files don't include private categories.
- Performance gains, as doc generation is quite slow.

One of the tests wasn't restoring the changes it made to the config.
As the test was difficult to track down and the problem would have
become only worse in the future, the issues was solved starting with
a new config before each run.

The performance impact seems negligible and the testing environment is
more predictable.

The actual warning:
./spec/unit/sandbox_spec.rb:4: warning: already initialized constant TMP_POD_ROOT
Pod::Sandbox

New methods
- #header_mappings_dir: specifies the folder from which to preserve the name-spacing
- #exclude_headers: specifies the headers that should be excluded by the header mappings.

Currently those methods are used by RestKit and SSZipArchive.

- #add_libraries
- #add_frameworks