Added USERENA_USERNAME_MIN_LENGTH and USERENA_PASSWORD_MIN_LENGTH options

[gsfakianakis]

I had a request for minimum username length, so I added the USERENA_USERNAME_MIN_LENGTH option in the default signup form.

I did the same for the password length and I added the USERENA_PASSWORD_MIN_LENGTH option.

[coveralls]

Coverage increased (+0.007%) to 95.588% when pulling ae4a8ca on gsfakianakis:master into 6cc0954 on bread-and-pepper:master.

[coveralls]

Coverage increased (+0.007%) to 95.588% when pulling 2dba684 on gsfakianakis:master into 6cc0954 on bread-and-pepper:master.

[coveralls]

Coverage increased (+0.007%) to 95.588% when pulling 792537d on gsfakianakis:master into 6cc0954 on bread-and-pepper:master.