Update installation docs

Created by: riyad

Requirements:

• reformatted/reordered
• add estimates for hard disk space
• add section for other Rubies

Database:

• clean up
• make both guides as similar as possible

Installation:

• fixed formatting
• add more explanations
• make warnings/important notes consistent
• remove unnecessary packages for building stuff, MySQL, PostgreSQL and Python (see 78d3611e for more details)
• update Ruby to 1.9.3-p327
• make sudo calls follow a consistent pattern
• only add gitlab user to git group
• fix Gitolite directory permissions
• remove duplication by referencing external documents (e.g. requirements or database config)

Please review this thoroughly. ;)