Xcode 5.1 fixes.

* Compiler started complaining about being unable to find inline function, so I
  just removed the inline keyword.
* XCTAssertEqual no longer handles structs.
* Bonus: delete unused header from HTMLTokenizerTests.m.
Update README alternatives and testing sections.

* New, popular libxml2 skin "Ono".
* Link to WebKit.
* Clarify which html5lib tests are run.
* Mention use in shipping app.
Test both iOS and OS X builds on Travis.

* Build, don't test, on iOS < 7 (we can't use XCTest that far back).
* Disable clang modules for OS X tests. Fixes complaints of "ambiguous
  expansion of macro 'NULL'".
* Set project-wide OS X deployment target to 10.7 so tests will run there.
* Update xctool before test run.
Avoid iOS7-only -[NSScanner scanUnsignedLongLong:].
Expose relevant API on earlier SDK versions.

* Expose -[NSArray firstObject] on iOS before 7.0, and OS X before 10.9.
* Expose subscripting support on iOS 5.
Add iOS/OS X test/build version coverage to README.