Done a new feature where allow global admins change the institution of a space

Before this feature, global administrators were unable to change the institution of a space after its creation. Global administrators should be able to change the institution of a space. So, to do that, an option with all insitutions was inserted into the edit page of a space (just visible to global administrators), and a guard function was created into the model to guarantee this action.

Fixes #1069
Done the tests to ensure the function of changing the institution of a space

The test consists create a new institution and update the attributes of a space to see if the institution was updated with the new institution. Was necessary the creation of a new type of space inside the institution factory.

Refs #1069
Improved some tests to ensure better quality

The tests are verifying if the number of spaces inside the institutions are consistent after the space change between institutions. For it was developed a new function inside the space model to set a specific institution for a space.

Refs #1069
Update space.rb

Forgot to make this guard to ensure that the new_institution isn't nil.

Refs #1069
Merge branch 'Feature1069' of git://github.com/fbottin/mconf-web into fbottin-Feature1069

Conflicts:
	app/models/space.rb
	spec/models/space_spec.rb
Adjustments after PR #3

- Cleaner input to set the institution in spaces/edit
- Fixed error when creating a space with an user that doesn't belong to an
  institution
- Added a few more tests