String constants and import obfuscation

The release includes the following improvements:

• String constants are obfuscated so they are unreadable and they are very hard to grep by
• Import instructions are obfuscated (import statements are translated to __import__ invocation which is wrapped into eval and obfuscated as a string).
• Fixed bug in lambda function processing. Now lambdas with default parameters don't cause the obfuscator to fail.