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.
First, .so.4 is a symlink as well. When dart-core is built, the following files are generated:
```
libdart-core.so (symlink)
libdart-core.so.4 (symlink)
libdart-core.so.4.2 (real library)
```

I thought libdart-core[MAJOR_VER]-dev packages host libdart-core.so.[MAJOR_VER], which are symlinks, and libdart-core[MAJOR_VER].[MINOR_VER] pacakges host libdart-core.so.[MAJOR_VER].[MINOR_VER], which is real libraries. 

Is it more standard if libdart-core[MAJOR_VER]-dev packages host libdart-core.so?