Linux support : added AtmelStudio7Generators support on Linux ?

[bebenlebricolo]

• Code is mostly, if not completely, platform independent and might run on Linux as well as on Windows
• Move

this->Generators.push_back(cmGlobalAtmelStudio7Generator::NewFactory());

in the linux || _WIN32 block in CmakeLib/cmake.cxx file

• Add AtmelStudio7Generators subdirectory in linux and windows builds in Source/CMakeLists.txt (for CMakeLib)

• Provide Windows/Linux platforms support to handle AtmelStudio7InstallationDir stuff with registerkeys, which are not available in Linux -> Provide a workaround for that if required

[bebenlebricolo]

Another big part of this is to decouple AtmelStudio7 tools from within the tests.
At the moment, generators are really tied to the CMake generator's architecture and as AtmelStudio7 is a variant of Visual Studio it made sense to derive from the VStudio generators.

However these choices make this project hard to test and validate, and we may benefit to create more realistic tests and look at the generated .atsln files.

Will open another ticket for that. Linked with #44