Fix issue with C headers not compiling in C++ programs

• https://github.com/fedora-modularity/libmodulemd/issues/30

• Fixes numerous memory-leak issues

• Revert backwards-incompatible change to nsversion for GObject Introspection
• Make default stream and profiles optional
• Fixes: https://github.com/fedora-modularity/libmodulemd/issues/25
• Fixes: https://github.com/fedora-modularity/libmodulemd/issues/26
• Fixes: https://github.com/fedora-modularity/libmodulemd/issues/27

• Adds support for handling modulemd-defaults YAML documents
• Adds peek()/dup() routines to all object properties
• Adds Modulemd.Module.dup_nsvc() to retrieve the canonical form of the unique module identifier.
• Adds support for boolean types in the XMD section