BERLIN (Reuters) - Support for Chancellor Angela Merkel's conservatives has nudged higher, an opinion poll showed on Sunday, but the weakness of two smaller parties highlights the trouble she may face building another center-right coalition after Germany's 2013 election. The Emnid poll conducted for the Bild on Sunday newspaper showed the conservatives up one percentage point at 36 percent but their junior coalition partner, the pro-business Free Democrats (FDP), down one point at 4 percent, below the 5 percent threshold for entering parliament. ...