Jun29 fix proposals

Changes:

1. Fixed all proposals specs.
2. Started using strong_parameters ( http://railscasts.com/episodes/371-strong-parameters ).
3. Changed the interface a bit for votes_attributes. Previously we used two ways of populating votes_attributes.  
     a) votes_attributes: { :comment => "Abc" }  
     b) votes_attributes: { "0" => { :comments => "Abc" } }  
    I have unified it so that we only use approach a.  
    Kim, you will need to make corresponding changes in Angular for those test cases where it changed from approach b to approach a.  Check: https://github.com/tispratik/spokenvote/compare/Jun29_fix_proposals?expand=1#L7L20