Added slices to the prime sieve.

The following syntax is now possible when using the prime sieve:

```python
from sympy import sieve

assert sieve[5] == 11
assert list(sieve[5:10]) == [sieve[x] for x in range(5, 10)]
assert list(sieve[5:10:2]) == [sieve[x] for x in range(5, 10, 2)]
```
Merge remote-tracking branch 'jackolantern/feature-ntheory-sieve-getitem-slice'

Conflicts:
	sympy/ntheory/tests/test_ntheory.py
Add John Connor to the AUTHORS/aboutus. Welcome to SymPy!