Since Ceres is moving to using GitHub for issues, and the Google
Code URL in the current copyright header will soon become invalid,
update all the headers.
Change-Id: I1fce70375d1bcf098591f07b4d8f01a5c1e0789c
1. Merge TrustRegionLoggingCallback and LineSearchLoggingCallback
into a single callback.
2. Move the callbacks into callback.h
3. Update SolverImpl to use the new callbacks.
Change-Id: I9e82173cf2b828d023d96c57d1cba17f4832aeae