Encapsulate all RSA operations in Key class

All basic signature definitions and operations are encapsulated in the
Key class. Default signature algorithm is defined to be RSA with 2048
bit keys and SHA1 digest. If these defaults are used, no other code
needs to know about any signature algorithm to:

- Generate Keys
- Encapsulate public/private keys
- Sign/Verify documents
Centrally define hashing algorithm

The hashing algorithm and the algorithm name used for creating
target/meta hashes is defined as constants in the TUF module and used
throughout the code for hashing operations.