Update README.

- Fix code highlighting
- Add a note about simplejson module before Python 2.6
- Minor reST markup corrections
Fix usage of equality operators for None.

That type of comparisons should always be done with
"is" or "is not", never the equality operators.
Use the isinstance() builtin function for type comparisons.
Merge pull request #5 from berkerpeksag/fix-none-usages

Fix usage of equality operators for None.
Merge pull request #6 from berkerpeksag/use-isinstance

Use the isinstance() builtin function for type comparisons.
Merge pull request #4 from berkerpeksag/readme-update

Update README.