Don't recompute the current overlay in mmm-set-local-variables

Instead pass it the current overlay as a second argument.

* Rename mmm-set-current-submode to mmm-set-current-pair, take overlay as the
  second argument.
* Use the above in mmm-update-current-submode.
* mmm-get-saved-local-variables: Take overlay as the second argument, move the
  filtering code to a new function, mmm-submode-overlay-at, use it in
  mmm-fontify-region-list.
* mmm-save-changed-local-variables: Reverse the arguments order, for consistency
  with other functions. Update callers.
* mmm-fontify-region: Save the current overlay instead of its beginning
  position, then restore it directly.
* mmm-make-region: Pass the newly created region to mmm-set-local-variables,
  instead of making the latter look for the former at point, like previously.
* mmm-make-region: Don't call mmm-save-changed-local-variables, or set the
  mmm-previous-* vars, the next successful call to mmm-update-submode-region
  will do that for us.
* mmm-set-local-variables: Default to mmm-primary-mode
Merge pull request #20 from dgutov/overlays

Some more cleanup around mmm-current-* vars
Use syntax-propertize-function set in submodes

WIP, it visibly slows down parsing.
Define mmm-syntax-propertize-function

This is faster, but still kinda slow.
* mmm-syntax-propertize-function: Use font-lock-syntactic-keywords
  with less fortunate submodes.
* Don't clear text properties, syntax-propertize already does that for us.
* mmm-apply-all: flush syntax cache
Revert c548593, for easier optimization

* mmm-fontify-region-list: Don't set global and buffer vars when the submode
  hasn't changed. Gives us nice perfomance boost.
Rewrite mmm-regions-alist for performance

Not much improvement here, but at least we don't iterate multiple times over the
same list now.
* mmm-regions-in: Keep overlay references in the return value
* mmm-fontify-region-list, mmm-syntax-propertize-function: Use them.
* mmm-erb-scan-region: Same.
* mmm-submode-overlay-at: Remove, not used anymore.

And bump the version, this is a change in the API.
Hopefuly, a relatively safe one.
Rebinding syntax-propertize-chunk-size is pointless

It's only used in the caller, to determine STOP.
Merge pull request #21 from dgutov/syntax-propertize

Define a composite syntax-propertize-function
* mmm-noweb.el: Add the FIXME note.
Make README more up-to-date
* mmm-noweb.el: Qualify the "doesn't work" statement.
Remove the angle brackets
Update the TODO
Add URL header
Some compatibiliy tweaks

* Prioritize syntax-begin-function over font-lock-beginning-of-syntax-function.
* Check if syntax-propertize-function is bound. It isn't in Emacs 23.