ISLAMABAD — Negotiators for thousands of protesters demonstrating outside of Pakistan's parliament met Wednesday with politicians trying to end the crisis, but key challenges appear to remain — including their demand that the prime minster resign. Anti-government protesters converged on Islamabad more than two weeks ago, demanding Prime Minister Nawaz Sharif step down. The twin protests are led by opposition politician Imran Khan and fiery cleric Tahir-ul-Qadri. The negotiations Wednesday signified a…