build: rename the pkg-config file to dbus-sharp-2.0.pc

The new name matches the new API version, and prevents us from
overwriting the .pc file from previous version.
build: Only have one AssemblyInfo file

Previously we had AssemblyInfo.cs generated from AssemblyInfo.cs.in by
the configure script, and used when building with make, and a
AssemblyInfoStatic.cs with hardcoded version numbers, used when building
with the csproj.

As the compilation is now done with xbuild, the one referenced from the
csproj is always used, which can cause some confusion.

We now reference AssemblyInfo.cs in the csproj, still generated from
AssemblyInfo.cs.in, to make sure version numbers are always up-to-date.
But we add it to git, so that you can use the csproj directly from a git
clone, without need to first run configure.

The unique place to set the version numbers is now configure.ac, and it
will propagate to all the right places.
build: Modernize some autotools macro calls

This avoids deprecation warnings with recent versions of automake and
autoconf.
build: Update the console output when running gacutil

This makes the echo'ed command line actually match the command that is
executed, but doesn't change the command itself.
Merge pull request #26 from bl8/parallel-install

Allow parallel install, and some build clean ups