Improvements to string generator

[thomasjthomasj]

This PR makes some changes to the string generator:

• Deprecates setPattern() and $_pattern property as it is too difficult and inefficient (and at times impossible) to create a string that matches a regular expression
• Adds setTenacity() to determine how hard the string generator should try to make the string that matches the pattern and/or whitelist. This is the number of times it will attempt to create a string that matches a regex, and within that generation, how many times it will filter out characters that are not in the whitelist. This defaults to 1000, which means that if there was a very strict regex and very limited number of characters in the whitelist, it could potentially loop a million times, however, since the regex functionality is deprecated, you shouldn't be using both features in tandem anyway.
• Adds allowChars() method for developers to set which characters should be in the whitelist. This will override the current whitelist
• Adds disallowChars() method for developers to remove characters from the whitelist.
• Adds Exception\GenerateStringException to be thrown when a string generation method has failed to create a string within the number of loops set by the tenacity
• Adds attempt for string generation with /dev/arandom before /dev/urandom when generating a string using generateFromUnixRandom()

Resolves https://github.com/mothership-ec/cog/issues/449

[kuiche]

There are a couple of coding points in generateFromUnixRandom() and generateFromOpenSSL(). They are essentially the same code, so can they match as close as possible. There are also a couple of places that need breaklines (between if and where statements).

[kuiche]

Use anon functions for different methods for hint reasons.