Adding in new version of LibGit2Sharp, 0.19.0.

As a result of some changes to the core LibGit2Sharp Library, in order to properly support Stash, I am suggesting that we pull in the latest version of LibGit2Sharp.
Merge pull request #248 from gep13/LibGit2Sharp0.19.0

Adding in new version of LibGit2Sharp, 0.19.0.
Update ReleaseNotes.md

Added information about v1.2.1. release
Merge pull request #249 from gep13/patch-3

Update ReleaseNotes.md
update to newest pepita
bump test refs
fix typo
make versioning scheme configurable

-moved the scheme logic into the task since it isnt a core concern
(perhaps the exe should make use of this in the future)
simplify assembly versioing
simplify tests
split out exe test
remove redundant module init
remove redundant code
rename acceptance tests to GitVersionCore.Tests

also make testing lib consistent
rename test project
move VariableProviderTests
move version tests
move git dir tests
move argument parser
move InformationalVersionBuilderTests