Move spectrum plugin from -bad.

Move spectrum plugin from -bad.

Original commit message from CVS:
* configure.ac:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* docs/plugins/gst-plugins-good-plugins.args:
* docs/plugins/gst-plugins-good-plugins.hierarchy:
* docs/plugins/inspect/plugin-spectrum.xml:
* gst/spectrum/Makefile.am:
* tests/check/Makefile.am:
* tests/check/elements/.cvsignore:
* tests/examples/Makefile.am:
Move spectrum plugin from -bad.
Move examples into tests/examples/spectrum.