Prepare version 0.8.0.

bump to v0.7.0

Merge pull request objectbox#102 from objectbox/dev

v0.6.4 release

Merge pull request objectbox#101 from objectbox/dev

v0.6.3 release

Merge pull request objectbox#96 from objectbox/dev

v0.6.2 release

Merge pull request objectbox#89 from objectbox/dev

v0.6.1 release

Merge pull request objectbox#86 from objectbox/dev

v0.6.0 Release

fix issue reference in the changelog

Merge pull request objectbox#60 from objectbox/dev

v0.4.0 release

update the v0.3.0 release date