MapDB 3.0 beta4 released

[Jan Kotek]

Hi,

there is new bugfix release 3.0.0-beta4

**FORMAT CHANGE**
POJO serializer format has changed. If you use default serializer together
with your own objects, your data might not be readable. Well known java
classes in ``java.lang`` such as ``String``, ``Long``, ``BigDecimal`` etc are
not affected.

Also there are bugfixes in BTreeMap, HTreeMap and DB.close().

Change log:
http://www.mapdb.org/changelog/

Regards,
Jan