Version numbering of the shared libraries in debian packages

Version numbering of the shared libraries in debian packages

This PR changes naming of the shared libraries. Since ABI breaking is done per minor version up, minor version number needs to be appear to the shared libraries.

For example:
* `libdart-core4.2` package installs `libdart-core.so.4.2`
* `libdart-core4-dev` package installs `libdart-core.so.4` symlink

This PR should fixes #281.
It's an clear explanation, thanks!

I made changes that applies the policy and the other comments of yours. Please let me know if it needs more change.