Add generic pbkdf2-hmac-sha1 format

[magnumripper]

Support hex encoding and variable number of rounds and output length.

We could use this output format:

$pbkdf2-hmac-sha1$iterations.salt.hash

Output length is not specified in a field but read from the hash. So it is part of the salt struct.

Example usage:
http://hashcat.net/forum/thread-2892-post-17493.html

IOS7 Restrictions password
From com.apple.restrictionspassword.plist:

    <key>RestrictionsPasswordKey</key>
    <data>
    J94ZcXHm1J/F9Vye8GwNh1HNclA=
    </data>
    <key>RestrictionsPasswordSalt</key>
    <data>
    /RHN4A==
    </data>

algo is pbkdf2-hmac-sha1 with an iterations count of 1000 and output length 20. Password for the above is 3956
Output from ios7rest2john for the above hash should be:

$pbkdf2-hmac-sha1$1000.fd11cde0.27de197171e6d49fc5f55c9ef06c0d8751cd7250

[magnumripper]

First version committed in 5bac56f (see also #465)