Error message grammar, punctuation, etc

I'm noticing a couple mistakes and inconsistencies in the error messages shown by this module:

• Perhaps this only applies to American English, so sorry if this is simply a case of my own ignorance, but the error messages for the max_length and max_words validators seem grammatically incorrect.

  Before: %item can be maximum %num characters long
  After: %item can be at most %num characters long

• There seems to be inconsistent use of "should be", "must be", "has/have to", etc. I tried to standardize this where possible.
• Most error messages do not end in a period. Drupal core's error messages do, and it seems like this module's should as well.

Patch forthcoming.