Removes whitespace tokens from the array ends
@param {Array} tokens
@param {Number} mask Mask indicating from which end whitespace should be
removed
@returns {Array}
function trimWhitespaceTokens(tokens, mask) {
		mask = mask || (WHITESPACE_REMOVE_FROM_START | WHITESPACE_REMOVE_FROM_END);
		var whitespace = ['white', 'line'];
		
		if ((mask & WHITESPACE_REMOVE_FROM_END) == WHITESPACE_REMOVE_FROM_END)
			while (tokens.length && _.include(whitespace, _.last(tokens).type)) {
				tokens.pop();
	 		}
		
		if ((mask & WHITESPACE_REMOVE_FROM_START) == WHITESPACE_REMOVE_FROM_START)
			while (tokens.length && _.include(whitespace, tokens[0].type)) {
				tokens.shift();
			}
		
		return tokens;
	}