Adds support for dependent link traversal.

Methods for doing dependent link traversal have been added. The link
traversal unit tests don't currently run properly. There is an issue
with thread synchronization and the unit test thread is not signaled
when the completion block of the traversal is done executing.
Tests all pass. **Timing Issue Remains**

All of the traversal tests pass. This commit marks a working version,
but with timing issues. It should not be relied upon until the timing
issue is addressed.

The approach taken for traversal right now relies on a
single NSOperationQueue and managing the dependent NSOperations may not
be entirely correct. A modified approach where the OHLinkTraverser
derives from NSOperation and maintains an internal NSOperationQueue
would allow for truly dependent child operations.
Fixes timing issue with dependent traversals.

Introduces a new class called OHLinkTraversalOperation that is
subclassed from NSOperation. This class is responsible for the
coordination of nested operations. The unit tests now all pass
as expected and the timing issues experienced earlier appear to
have been resolved.
Fixes build issue with xctool version on Travis-ci
Updates podspec and removes outdated files.
Merge pull request #13 from ObjectiveHAL/feature/add-link-traverser

Feature/add link traverser