Add a new api for simple encrypt and decypt

Adds a new api which can be used to decrypt and encrypt
small amounts of data. The api looks like:

```java
byte[] cipherText = crypto.encrypt(plainText);
byte[] plainText = crypto.decrypt(cipherText);
```

Test Plan:
Wrote integration tests, they pass.
So do all the unit tests.
Address comments

Test Plan:
Unit and integration tests.
Add description for exceptions

Test Plan:
UNit and integration tests.
Merge pull request #15 from facebook/add_new_api_2

Add a new api for simple encrypt and decypt