Miscellanous fixes in preparation for 1.6.0

1. Bug fix in NumericDiffCostFunction (Thanks to Nicolas Brodu).
2. Minor documentation update in solver.h
3. Version history update.
4. Bump the version and ABI version.

Change-Id: I951574ddd0b2c4c03b9c79ff33eb9bea549071e7
5 files changed