add acccessor_locales option to create methods for setting/getting translations in a given set of locales, for issues like #275.
check that translation accessors are created only for locales passed in as option to 'translates'.
use :key => value format so code works with ruby 1.8.7.
only accept array values for accessor_locales option.
add explanation on use of accessor methods to readme.
Merge pull request #277 from svenfuchs/translation_accessors

Translation accessors