Add failing tests involving HTML parsing

These tests exercise various aspects of HTML handling within markdown
documents. They will be the basis of the 0.3 release.
Fix a few small warnings related to format strings

* In 2 cases, %u was being used with NSUInteger, which is an unsigned
  long on 64-bit systems. Cast the value to (unsigned int), since the
  expected values only go up to 6.
* nil was used erroneously at the end of an argument list for one
  format string.
Begin to actually parse HTML

Introduce a new class, MMTHMLParser, to handle the parsing. It will
only handle a subset of valid HTML, but it's enough to begin passing
some failing tests.

MMSpanParser now tries to use this class to parse inline HTML. By
doing so, it will correctly handle attributes which look like they
contain markdown, such as this:

    <a href="#" title="*Bold Title*">A Link</a>

In such a case, the asterisks should be passed through unchanged.
Fix an incorrect HTML test by doing a string comparison

The test was written incorrectly before. After correcting the test by
dropping the double-quotes, it was no longer valid XML. So the existing
test function, MMAssertMarkdownEqualsHTML, would pass because both the
generated HTML and the expected HTML would fail to parse and return
nil.

Introduce a new test function, MMAssertMarkdownEqualsString, which
performs a string comparison on the actual and expected outputs.
Add support for alternate ASCII line-endings

The scanner was assuming that all lines will end with \n. But the world
is not so simple. Add support for \r and \r\n line endings along with
tests that verify the behavior with them.
Fix #1: List items weren't handling leading spaces

Levels in a list are determined by the number of leading spaces (4 per
level). But each item can also have up to 3 leading spaces. The code
that parses each list was allowing for this, but the code that parses
each item was not.
Update lib/ files and release notes from 0.2.1 release
Update project to Xcode 4.5 recommended settings
Update lib/ files and release notes from 0.2.2 release
Merge changes from 0.2.3 release
Fix crash when passed an empty string.
Add a test for empty input strings to go with #4
Update to Xcode 4.6.2 recommended project settings.
Fix the handling of newlines in between HTML attributes
Add more lenient support for HTML attributes and values
Remove some failing tests for behavior that's not standard
Move code to parse block HTML tags into MMHTMLParser
Change the style of method declarations
Remove @synthesizes since they're no longer needed
Fix handling of value-less HTML attributes