fix :SuperTabHelp to not be dependent on &cb

fixes #72
remove <c-n>/<c-p> mappings

fixes #70
add <silent> to <c-x> and <cr> mappings

fixes #73
return <tab> if backward mapping is set to <tab>

If either the forward or backward supertab mapping is set to <tab>, then
we should return a <tab> when not completing.

fixes #77, #75
add support for delegating to the smart tabs plugin

fixes #78
add a couple FAQs to the README
install and uninstall targets
don't map <cr> if user has an iabbrev w/ a <cr> in it
switch <c-x><c-p> / <c-x><c-n> completion in <c-p> mode
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