sieve of eratosthenes implemented

[prachiagrawal269]

References #4
Please review

[aktech]

@prachiagrawal269 Please fix conflicts.

[goelakash]

@prachiagrawal269 Please fix conflicts and squash commits.

[goelakash]

@aktech I was thinking if this should be optimized. Checking for even non-primes first, lets us use increments of 2*i instead of i (as all even numbers are filtered beforehand).
For eg:

not_prime[2] = False
for i in range(3,lim):
    if i%2 == 0:
        not_prime[i] = True

for i in range(3, lim):
    if not_prime[i] == False:
        for j in range(i*i,lim,2*i):
            not_prime[j] = True

[tapaswenipathak]

#12 too.