don't mark first comment as error, and rx cleanup

  The previous version of `git-commit-summary-regexp' highlighted the
  first comment line as a git commit style error.  This was because it
  did not check if bad non-empty second lines were actually comments.
  In addition this commit removes some messy line-start and grouping
  constructs which were not necessary.
customizable max summary length and remove `rx'

No longer using the `rx' package.  Introducing a new defcustom named
`git-commit-max-summary-line-length' to control the max summary line
length.  When this variable is set, an initialization function re-builds
the `git-commit-summary-regexp'.
Merge pull request #38 from eschulte/master

don't mark first comment as error, and rx cleanup