I've merged the "proposed-plugin-api" branch into "master"

I'm working on adding support for using the plugin with gcc 4.8 prereleases, and so it was time to "bite the bullet" and merge the "proposed-plugin-api" branch. This adds a "gcc-c-api" subdirectory: an intermediate C layer that's intended to shield the plugin from changes to GCC's own API (and hopefully eventually become an API for all GCC plugins to use). The 4.7 vs 4.8 differences can live there. I also plan a big symbol renaming: given that all of the gcc-c-api's symbols begin with gcc_, it's somewhat confusing to have symbols within the plugin proper that use such naming. So I plan to rename plugin symbols so that they are prefixed with PyGcc and use CamelCase_WithOneUnderscore, like Python itself. (this is a slight violation of PEP 7 in that you're not meant to use "Py" as a prefix, but I don't see it being an issue). This should make it easy to distinguish the gcc-c-api layer (lower_case_with_multiple_underscores) from the python plugin layer (CamelCase_WithOneUnderscore). Dave