generate 2048-bit RSA keys by default. 1024 bits is too weak.

I think mailvelope should encourage users to use at least 2048-bit RSA encryption, because there are reports that RSA-1024 can be decrypted by some agencies. NIST recommends using at least 2048-bit and has deprecated 1024-bits.
See page 5 of http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf

I would also propose removing the 1024-bit option from the "Generate Keys" page, but I haven't included this in this pull request.