DDFileLogInfo: updating extension methods for iPhone simulator
DDFileLogger: handling error with 'archived' extension in isLogFile: method (on iPhone simulator)

Fixes #215.
Refactoring DDLog: changing comments to NSAssert.

All 'lt_' methods should be executed only on logging queue. This was mentioned in comments
before each method. Now explicitly checking it with NSAssert.
Merge pull request #221 from dvor/ddlog-logging-queue-asserts

Nice!
Merge pull request #218 from dvor/issue215-bug-with-archived-logs

Fixed bug with archived logs
Bumped version to 1.8.1
Updated README.markdown