i sympathize with you, i'm 5' 7" and 111 lbs and yeah i can do the thumb/middle finger-round-the-wrist thing too. everyone in my family skinny. i've been lifting 6 years and gained little. But whatever i do gain, it shows.
i've tried the eat-like-a-pig 'gaining' diets and all they did was give me a gut. it didn't matter how much or little protein it was or carbs, too many calories made me put on fat and didn't make me any stronger any faster. your body will only put on so much muscle at a time and will store extra as fat.
i would recommend upping the calories little by little (100 or 200 calorie additions) over the weeks, while keeping close tabs on your body weight and of course the lifts. When you see the scale going up, stay at that calorie level and see if your lifts improve. if you notice fat coming on, pare it back just a tiny bit. you might get a little bit of fat no matter what but as long as most of your gains are muscle you're on the right track, you can diet off fat later. i'm what they call a hardgainer i'm lucky if i go up one REP per week on my lifts, no matter WHAT i eat.
if your lifts have stagnated, that might be a sign you're not eating enough (it could also mean you need to change your routine), but if you're making any gains i wouldn't fix what isn't broken. Just telling you what my experience with this stuff has been.
food...
what i eat, I eat a can of tuna on most days, but i like to vary my protein--salmon is nother fish that's good, i have that once in a while, i believe it's much lower in mercury than tuna but i think it has higher calories too. Egg whites of course--very good source--whey protein, chicken, turkey, lean red meat and skim milk are other ways i get protein. for carbs, i do whole wheat bread, oat meal, cheerios and of course lots of veggies and some fruit. i try to get near 1 gram of protein per lb. of body weight. you're a guy though and i heard guys need a little more protein than women, so you could try more, i don't know.
but one thing it sure helps to do, is cut out all the cookies and candy and other sugary stuff. Except of course a treat once in a while ^_^