Unify tags

[vboitsov]

1. No Special Characters
   Hashtags only work with the # sign. Special characters like "!, $, %, ^, &, *, +, ." will not work.
2. Don't Start With or Use Only Numbers
   Hashtags like #123 won't work, so don't use only numbers. Similarly, #123yo doesn't work. But numbers are great for recurring events like #conference2012 or #SXSW12.