Merge pull request #1204 from lambday/feature/log_determinant

minor documentation fix in log-det framework
refactor evaluation of posterior and predictive distributions
Merge pull request #1206 from votjakovr/feature/gp_refactoring

refactor evaluation of posterior and predictive distributions
Multi-threaded dense_dot in HashedDocDotFeatures
Merge pull request #1207 from van51/feature/hashing

Made HashedDocDotFeatures thread-safe (reentrant)
bugfixes in log-det framework