Fixed several minor aesthetic problems with lein.el and marked some bigger problems with FIXMEs.
A wide array of small to moderate changes, including a reorganization of toplevel forms into sections. (see more)

- Added a contribution note in the toplevel comments.
- Grouped all toplevel forms into modular sections based on their purpose.
- Tweaked contents and formatting of various messages to read more clearly and consistently.
- Rearranged lein-command-string (now lein-task-command) for simplicity and readability.
- Checked for live buffer in lein-server-sentinel and simplified buffer-substring to buffer-string.
- Removed default-directory shadowing when composing the lein-launch-command.
- Numerous improvements to lein-server-filter, including:
  - checking for a live buffer.
  - managing the process-mark and point explicitly.
  - matching the nREPL server port even if process output comes
    in unexpected chunks (previous approach was brittle).
  - preventing repeated searching through process output for
    nREPL server port once it has been located.
  - removing unnecessary save-window-excursion.
  - fixing the scope bug (let -> let*) that prevented
    lein-words-of-inspiration from appearing in the minibuffer.
  - hiding the *nrepl* buffer after it pops up.
- Suppressed lein-launch output at the eshell prompt (restricted to minibuffer).
- Commented out the (eshell-remove-process-entry entry) line since entry appears to be unbound.
- Made a call to eshell/lein with no arguments resolve to the "lein help" task.
Merge pull request #7 from lambdatronic/master

Now lein.el rocks more than ever!