Fix wrong model recognition

Parser was recognizing words with `model` keyword combination, for example `` `model` `` or `Cmodel`, as model, this is a suggestion to fix the issue. this has been mentioned in both issue #152 and #92.
for not recognizing cases like `+ named model with something after keyword` we should use `$` at the end of regex to ensure nothing else would come afterward but on the other hand in testing I noticed for example in [here](https://github.com/apiaryio/snowcrash/blob/master/features/fixtures/blueprint.apib#L12) what comes as `subject` to this function is `+ Model/n<resource model description>/n` which then gets rejected because it does not match the regex, it's possible by adding some code to regex to prevent this line but it would make the regex line much complicated than it's already is.