Add option to create Letterblock passwords

Blocks of letters are easier to type than delimited word style passwords — simply because they require less letters — while being similarly easy to remember.

This merge request adds radio buttons to use blocks of letters. For details on this password style see http://www.draketo.de/english/secure-passwords (which also provides a JS implementation).

With extended delimiters, three blocks of 4 letters suffice to follow SCC words (80 bit of entropy). Note that there are slightly fewer extended delimiters than with delimited words because I thought some of them to be too easy to mix up. One Example:

7pTp.0si2|xbQw

To test, just run ./passwordGenerator.py