Remove 1.4.9999 ebuild
Update ebuild for new 1.5.1 release

* Update ebuild for new 1.5.1 release
* Add USE=mysql and USE=postgres
* Remove USE="luajit-bundled"
* Fix FEATURES="test"
* Fix libunwind dependecy
* Fix docbook generation
Add tarantool-1.5.1_p26 snapshot