Ogre importer cleanup, improvements and bug fixes

Hello Assimp devs,

I work on a project that uses Ogre. This naturally means that we don't need Ogre imports via Assimp, but we do use your library for other things. I wanted to chip in and give something back to your awesome project. Ogre being close to my heart I looked at the Ogre importer code you have. The code did it's job but it was pretty horrible to read through. The formatting, variable names, code comments were all over the place... well anyways I started cleaning it up for fun and ended up going through the whole importer.

* Major source code cleanup. Now imo it is easier to read and figure out.
* Added documentation and `@note`s for things that seemed fishy and should be fixed/looked at at some point.
* Documented Ogre related Assimp imported configs correctly and with clearer language.
* Renamed `.hpp` to `.h` as seems to be your convention.
* Bugfixes to material parsing.
* Probably some performance improvements for the whole importer. So many for loops and vectors :)
* More descriptive `DeadlyImportError` exception messages on what went sideways.
* More debug logging for people who want more info what the importer actually did.
* Added include guards to headers and `ASSIMP_BUILD_NO_OGRE_IMPORTER` checks to source code.

If you want to merge this in before the release is up to you. I tested the code with all the Ogre assets in your repo and it looks to work just like before.

My end goal for this cleanup was that I would like to make the Ogre importer better, adding features and I've been pondering of writing a Ogre binary mesh parser as well. I have done one in JavaScript so I would recon I could pull it off with C++ as well :)

**P.S.** In my commit messages I claim that the skeleton parser was broken, it really was not that was my error and I verified it works also on `master`. Me being a Assimp newbie I has pre transform vertices enabled that removed the correctly imported anims :)