(#142) Adding appveyor.yml

- Using template appveyor.yml from gep13.io repo, created new appveyor.yml file
- Simply calling build.bat, passing in PackageSolution as action to execute
(#142) Adding missing files in Tools folder

- Since by default, all exe's and dll's are not included in Source code, necessary files not checked in
- Add to do git add --force
(#142) Adding try/catches

- In an attempt to correctly report an error in psake script, adding try/catch blocks
- Also, use the psake variable to return the correct exit code
(#142) Adding artifacts to appveyor.yml file

- So that we can take a look, if required, at the generated files from the build, let's add them as artifacts.
(#142) Changing the way GitVersion is called

Based on suggestion here:

https://github.com/Particular/GitVersion/issues/254

Using ```/output buildserver``` to force download of all branches.

Having to call it twice, as I now need to call it once to force download of branches, and another time to get access to the LegacySemVer variable.
(#142) Changing path to NuGet.exe

- Based on this conversation:

http://help.appveyor.com/discussions/problems/791-any-thoughts-on-why-i-am-getting-an-error-in-build

Changing path to NuGet.exe to use \ rather than /
Merge pull request #143 from gep13/feature/Issue-142

(#142) Adding appveyor.yml