updates to supertab docs

- wrap code examples in vim's help file code snippet chars
- elaborate completion chaining docs
- add another custom context example
- other minor formatting changes
add support for honoring NoCompleteAfter on bs while in completion mode

refs #101
fix minor typo in README
init supertab buffer vars on manual completion enter

fixes #105
bump the version to 2.1

closes #104
use keepalt + bufhidden=wipe instead of delete for SuperTabHelp

- use keepalt to preserve the current file's alt file name
- using wipe to cause the alternate file name to be cleared in other
  cases so it doesn't remain as the now removed buffer name.
Fix minor typo in README
add an installation section to the README

closes #108
remove extraneous period from README
de-indent the code blocks in the README

when indented they start looking more like quotes with a leading gray
bar.
fix key capturing to handle expr mappings when possible

fixes #114
Add some references to the help
Merge remote-tracking branch 'blueyed/doc-fixes'
CompletionReset: feed the original BS key with mapping enabled

Backspace might have been mapped by the user, and this should be
executed.
SuperTabCodeComplete: handle dict return value from Func

If the first function in the chain returns a dict, instead of just a
list of words, look at (the length of) results.words.

NOTE: if results.refresh is set to "always" (as with YouCompleteMe), we
might want to return the empty list anyway, which would allow the
function to stay in the loop.

> || (has_key(results, 'refresh') && results.refresh == 'always')
Merge remote-tracking branch 'blueyed/fix-bs-feeding'
Merge remote-tracking branch 'blueyed/fix-chaining-with-dict-results'
fix undefined variable 'patch32'
fix spelling and whitespace in docs
add support for triggering user's <s-tab> when not completing

fixes #122