Added test to check what happens if there is a redundant next version.txt file
Passed in working directory to NextVersionTxtFileFinder.cs so it uses the same directory as the rest of the program
Changing currentDirectory to workingDirectory in GitDirFinder to reflect the fact it might not be the current directory
Fixing spelling mistake in NextSemverCalcualtor
Fixed up and made consistent all of the tests that need a NextVersion.txt file
Merge pull request #13 from robdmoore/acceptance-tests

Acceptance tests