#155 Resolves repository location based on pdb path

[AmadeusW]

Fixes #155

Changes proposed in this pull request:

• Finds the repo root based on the pdb path. Previously, repo root was resolved based on an arbitrary file, which might have been located outside of the repo, e.g. in the global .nuget cache directory
• Consolidates code such that there is only one way to resolve repo root and the error message is descriptive and helpful
• In Readme, explains how repo root is located and explains the -baseDir parameter
• Also improves the error messages when libgit2sharp fails to initialize.

Checklist

• [X] I have included examples or tests (improved readme)
• [ ] I have updated the change log (where is it?)
• [X] I am listed in the CONTRIBUTORS file
• [X] I have cleaned up the commit history (use rebase and squash)

[GeertvanHorrik]

Looks really good, thanks!

[GeertvanHorrik]

Just pushed a new alpha build to both NuGet & Chocolatey.