Improved platform naming in package filenames.

The SCons PLATFORM variable isn't very user friendly - "darwin" for OSX and "posix" for Linux. This introduces a new GAFFER_PLATFORM variable with values "osx" and "linux" and uses them when building release packages.
Added additional types to the CompoundDataPlug new plug menu.

Fixes #522.
Added python subclassing ability to Serialisation::Serialiser.

Fixes #520.
Fixed bug in searchable Menus with no entries.

Fixes #527.
Merge pull request #529 from johnhaddon/pythonSerialisation

Added python subclassing ability to Serialisation::Serialiser.
Merge pull request #530 from johnhaddon/emptyMenu

Fixed bug in searchable Menus with no entries.
Merge pull request #525 from johnhaddon/compoundDataPlugMenu

Added additional types to the CompoundDataPlug new plug menu.
Merge pull request #524 from johnhaddon/packagePlatformName

Improved platform naming in package filenames.