Increase password security

[spieker]

https://github.com/spieker/pink1/blob/master/app/models/user.rb#L19

Default rounds are 5000, I should increase it!

http://blog.codinghorror.com/your-password-is-too-damn-short/

Example: UnixCrypt::SHA512.build('Some secret password', nil, 86000)

[spieker]

In order to prevent DoS, we should limit the maximum password length to 100.

puts Benchmark.measure { p = 'x'*6; 100.times { UnixCrypt::SHA512.build(p, nil, 86000) } }
# 37.130000   0.000000  37.130000 ( 37.156434)

puts Benchmark.measure { p = 'x'*100; 100.times { UnixCrypt::SHA512.build(p, nil, 86000) } }
55.580000   0.000000  55.580000 ( 55.613038)

This was run on a small Droplet at DigitalOcean.