Handle array column limits

Revise the warning in the README

See #122. I've revised the wording to placing the important text in the first sentence, and made the warning a bit more 2013-like by referencing VCS instead of backing files up.

if column limit is an array, the values are serialilzed into column attr
  cf enum_column3 gem

add support for composite_primary_keys

Fix README with correct generator syntax

Fix position option

run rake gem:gemspec to fix binary references

restore PR#50 - see https://github.com/ctran/annotate_models/commit/cfeec96c9ca0fa5035b10be3d73e798cc4fc52f7

Update README.rdoc

Corrected rails generate command to reflect the current generator name: annotate_models:install

Fixed the invalid gemspec error