Clarified emph/strong rules.

Make it clear that in `**foo **bar baz**` the shorter of the two
potential strong emphasis spans (`bar baz`) is preferred.
Update spec version.
Rename number_of_emphasis_openers -> emphasis_nestlevel.
Changed EMPHASIS_STACK_LIMIT -> STACK_LIMIT.

We'll also use it in parsing bracketed link labels.
Disallow bracketed labels nested more than 1000 deep.

For performance reasons.  See #166.
Added OPTCFLAGS, OPTLDFLAGS
js/lib/blocks:  Made matchAt return -1 instead of null on no match.
Typo in runtests.pl
Add -fPIC to linker options.

Fixes #179.
Update .gitignore and fix Makefile clean target.
Merge pull request #181 from michaelsproul/fpic-linker

Add -fPIC to linker options.
Merge pull request #182 from michaelsproul/cleanup

Update .gitignore and fix Makefile clean target.
Merge pull request #178 from jmendeth/patch-1

Typo in runtests.pl
Removed artificial rule for emph/strong markers.

Previously there was a rule that nothing in a string of more than 3
`*` or `_` characters could close or start emphasis. This was artifical
and led to strange asymmetries, e.g. you could have
`*a *b**` emph within emph but not `**a **b****` strong within strong.

The new parsing strategy makes it easy to remove this limitation.

Spec, js, and c implementations have been updated.  Spec might need
some further grooming.
Update spec version.
Made make_site_index.sh more robust.
Rewrote emph/strong part of spec, with more systematic examples.
Bump version on spec to 0.9.
Updated README to indicate where to get prebuilt commonmark.js.