[Resolves #14] Add console info about tenants and fast switches

[rpbaltazar]

• Adds console methods for:
  • tenant_list - listing all tenants based on Apartment.tenant_names
  • st(tenant_name:String) - switch to tenant if it exists
• Adds custom pry console that shows the current tenant name and rails env. Depends on pry-rails gem.

[rpbaltazar]

@rjayroach if you have some time, please take a quick look at this. I think this solves the issue and we don't need to release a separate gem only for this. If it becomes a more complex thing we can consider extract it to a separate gem.

The failing spec will be sorted out once I drop the support for rails 4.2 (#11)