Adding option for more secure Bcrypt hashing

Option to enable more secure hashing using Bcrypt. 
Defined in app.js bysetting "strong-slow-crypto" = true

Depends on: 
- node.bcrypt.js (through npm)
- openSSL should be installed on system

Rationale: 
Although the currently used sha512 is a good hash-function it doesn't protect againt ranbow-attacks once the database is compromised (in which case the app-code is likely compromised too, making the static salt exposed).

Brcypt basically does 2 things to protect against such attacks:
- A per-user salt is formatted into the hash.
- The hash function is sloooow