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.
Two thoughts:

1- About SONAME used

When breaking ABI on minors, there is no point for me on having a SONAME (which will be stored in applications using the library) defined only by majors (libdart-core.so.4) since the next version 4.3 is not going to be compatible with 4.2. So, the SONAME should be lib.so.{major}.{minor} and our library real name could be lib.so.{major}.{minor}.{patch}.

2- About packaging

From the [debian policy](https://www.debian.org/doc/debian-policy/ch-sharedlibs.htmle):

`Shared libraries are normally split into several binary packages. The SONAME symlink is installed by the runtime shared library package, and the bare .so symlink is installed in the development package since it's only used when linking binaries or shared libraries.`

So, we should put .so inside -dev and the .so.Major.Minor (symlink) and .so.Major.Minor.Patch (binary) inside the lib package.

Sorry for the previous confusion, let me know if I need to explain something better.