Bug 928195 - Part 3: Consolidate all WebIDL Python logic into mozwebidl module; r=bz, froydnj

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Bug 928195 - Part 4: Rewrite WebIDL build system integration; r=bz, r=glandium

WebIDL build system integration has been rewritten from the ground up.
Changes:

* GlobalGen.py, BindingGen.py, and ExampleGen.py have been removed in
  favor of mozwebidl.py.

* Static .webidl files are now processed directly in their original location
  and aren't copied to the object directory.

* Generated events <stem>.cpp files are now compiled into the unified
  sources. Previously, only the <stem>Binding.cpp files were compiled
  into unified sources.

* Exported .h files are now generated directly into their final location.
  Previously, they were generated into the local directory then
  installed in their final location.

* The list of globalgen-generated files now lives in Python and isn't
  duplicated in 3 places.

* The make dependencies are much simpler as a result of using a single
  command to perform all code generation. The auto-generated .pp file from
  code generation sets up all dependencies necessary to reinvoke code
  generation and Python takes care of dependency management.

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extra : rebase_source : e4918878274b22a412329c7cb18cc7138daf5dc6
Bug 928195 - Part 5: mach command for generating WebIDL example files; r=froydnj

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extra : rebase_source : c74d5f9556ca866622b7fefd4711e96fe4e4e020