PKCS#1 vectors

From ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/ and
ftp://ftp.rsa.com/pub/rsalabs/tmp/
Merge pull request #537 from public/pkcs1-vectors

PKCS#1 vectors
Fixes #539 -- document which interfaces a backend implements
Complete sentences.
Merge pull request #540 from alex/document-interfaces

Fixes #539 -- document which interfaces a backend implements
Handle multiple errors on the stack
Merge pull request #541 from public/fix-466

Handle multiple errors on the stack
Include the fernet vectors in the sdist

We include the other vectors, so consistency!
Merge pull request #542 from alex/include-fernet-vectors

Include the fernet vectors in the sdist
Link to our issue tracker in the readme
Merge pull request #544 from alex/issues-in-readme

Link to our issue tracker in the readme
More detailed error message for InternalError

Uses ERR_error_string_n because the source contains horrible warnings
against ever using ERR_error_string.
Merge pull request #545 from public/error-strings

More detailed error message for InternalError
Only put a single top level header in this page
Merge pull request #546 from alex/cleanup-index

Only put a single top level header in this page
Rename RSAs key_length to key_size

So that it matches the existing documented CipherContext stuff.
Merge pull request #548 from public/rename-key-length

Rename RSAs key_length to key_size
OpenSSL BN math

Needed to fully populate e.g. backend RSA contexts from an
RSAPrivateKey.
Merge pull request #547 from public/bn-maths

OpenSSL BN math
Refer to the `d` param of RSA as `private_exponent`.