fix(pagination): use interpolation for text attributes

Closes #696

BREAKING CHANGE: The 'first-text', 'previous-text', 'next-text' and 'last-text'
  attributes are now interpolated.

  To migrate your code, remove quotes for constant attributes and/or
  interpolate scope variables.

  Before:

  <pagination first-text="'<<'" ...></pagination>

  and/or

  $scope.var1 = '<<';
  <pagination first-text="var1" ...></pagination>

  After:

  <pagination first-text="<<" ...></pagination>

  and/or

  $scope.var1 = '<<';
  <pagination first-text="{{var1}}" ...></pagination>