Library is broken on Mac OSX

```
When using the library on Mac OSX, most things will fail because the library
tries to PInvoke to libdl.so, which does not exist on OSX. On OSX, dload() and
friends are provided by libSystem.B.dylib.

I've attached a patch that adds the necessary logic.

I've also attached some changes that make the build system and unit tests pass
on OSX. The primary changes are using Path.Combine to concatenate path
fragments rather than assuming any particular character, and the introduction
of a silly little bash-batch polyglot application to ensure that
AssimpNet.Interop.Generator.exe is invoked via mono.
```

Original issue reported on code.google.com by `andy.fri...@gmail.com` on 30 Aug 2014 at 8:54

Attachments:
- [0001-Add-OSX-support-dload-and-friends-are-found-in-a-dif.patch](https://storage.googleapis.com/google-code-attachments/assimp-net/issue-22/comment-0/0001-Add-OSX-support-dload-and-friends-are-found-in-a-dif.patch)
- [0002-Make-unit-tests-run-on-OSX-Rely-on-Path.Combine-to-k.patch](https://storage.googleapis.com/google-code-attachments/assimp-net/issue-22/comment-0/0002-Make-unit-tests-run-on-OSX-Rely-on-Path.Combine-to-k.patch)
- [0003-Tweak-post-build-action-to-work-on-OSX-CLR-executabl.patch](https://storage.googleapis.com/google-code-attachments/assimp-net/issue-22/comment-0/0003-Tweak-post-build-action-to-work-on-OSX-CLR-executabl.patch)

Start with the attached 0001, 0002, and 0003 patches, then inspect the P/Invoke entry points for dload() and friends, the unit tests, and AssimpNet.Interop.Generator.exe invocation. Done means the library resolves dynamic-loading functions on OSX and the build and unit tests pass there.

Written by the indexing model from the issue text.