improved generateString: added presets for character sets

Turns the second parameter $characters into a mixed parameter. When passing in an Integer it will be treated as a flag. Relevant flags are available within the Generator class as constants.

Example:
```
$generator->generateString(16, Generator::LOWER_CASE | Generator::DIGITS);
```

Yields something similar to:
```
yofch5qkv2nx0oay
```
Fixed some spelling issues.
Fixed some spelling issues #2
Code clean up & performance

Refrained from building character sets on-the-fly. Hardcoded every
character set (should be slightly more performant). Dropped usage
of underscores on variables. Commented the code better.
Merge branch 'patch-2' of https://github.com/flip111/RandomLib into flip111-patch-2
Resolving #15, cleaning up and refactoring to simplify and expand possible combinations. Test out the wazoo
Fix stupid missing variable error. Doh
Remove high byte generator, as it's basically useless if using anything but ascii
Merge pull request #24 from ircmaxell/improved_string_presets

Improved string presets