Issue #1: Add support for user-defined branch prefix to configuration

 * Added documentation for git-branch-prefix.
 * Added initial icon.
 * Added tests for git-branch-prefix.
 * Updated README to include details about git-branch-prefix.
 * gitignore
   * Added test-repo.git.
   * Added test/.gitignore.
Merge pull request #15 from jlmorgan/feature/issue-1

Issue #1: Add support for user-defined branch prefix to configuration