Fix for preposition vocalization: "v tři" -> "ve tři"

Will only use "v tř..." for some town names (where it is usual
according to the Czech National Corpus).
Merge pull request #15 from UFAL-DSG/nlg_vocalization_fix

A tiny fix for preposition vocalization: "v tři" -> "ve tři"
Merge branch 'master' into navigation_fixes