Refactor hook install and uninstall code

On hook uninstall move original hook back into original place (if it
exists).

Add notes about original hook to readme.
Output a red message on SprintlyException
Raise uncaught fatal exception for stack trace

Also emit a red error message
Rearrange readme slightly
Move commit hook logic into main module
Move call to initialize to the constructor
Tweak comments
Refactor commit hook, new features

- Allow Sprint.ly's alternate ticket syntax (ticket:44, issue:44,
  item:44, bug:44 as well as #44)
- Enter no string at all to skip referencing an issue, rather than 0
- Allow multiple item numbers at the start of the commit message
- Change default template to put the reference at the end
- Configurable commit message template via git config
  sprintly.template and sprintly.itemkeyword