Bump version to 1.6.8, add release notes

Add missing attributions for 1.6.5 release notes

Bump version to 1.6.6

Fix language code for Chinese localization

Bump version to 1.6.4

Add release notes for 1.6.3

Bump version to 1.6.2

Use GUID identifier when can't determine the host name

Merge pull request HangfireIO#614 from HangfireIO/dev

Release 1.6.0

Add project-specific release notes for 1.5.9