Cache pub/pri keys on retrieval

[simo5]

Pyca rightfully performs consistency checks when importing keys and
these operations are rather expensive. So cache keys once generated so
that repeated uses of the same JWK do not incur undue cost of reloading
the keys from scratch for each subsequent operation.

with a simple test by hand:
$ python

from jwcrypto import jwk
def test():
... key = jwk.JWK.generate(kty='RSA', size=2048)
... for i in range(1000):
... k = key._get_private_key()
...
import timeit

Before the patch:

print(timeit.timeit("test()", setup="from main import test", number=10))
35.80328264506534

After the patch:

print(timeit.timeit("test()", setup="from main import test", number=10))
0.9109518649056554

Resolves #243