Support providing a certificate chain and a single non-CA certificate.
New PKCS12 features and other fixes. Closes #13.

- Add MAC data to PKCS12 objects.
- Added 3des algorithm option and configuration options to pkcs12.toPkcs12Asn1().
- Refactored bags-to-SafeContents mapping so that each bag has its own SafeContents.
- Added localKeyId attribute to correlate key and certificate. Note: This may
  change in the future to be a hash of the key or similar (instead of random
  bytes).
- Added option to generate random localKeyId and option to turn off MAC; this
  also allows current unit tests to pass.
- Some minor code fixes and style changes, removal of special UTF characters, etc.
Remove special characters.
Use OID string printout when pretty printing.
Ensure PKCS12 certificates are digested when parsed.
Display more tag class information when pretty-printing.
Cache tbsCertificate for re-exporting/signature verification.
Add support for exporting certificate chains to PKCS#12.
Add PKCS#12 MAC verification.
Handle mixed types for validity times.
Fix toDer() for BMPString. Thanks @AchimO. Closes #16.
Only send ServerKeyExchange message if non-empty. Closes #17.
Check for extensions properly in hello messages. Closes #18.

- Keep track of the number of remaining bytes in hello message
  itself rather than looking in the buffer which may contain bytes
  from other messages. If bytes remain after parsing the main
  hello message, then check for extensions.
- Thanks to @Ayms and Tom Mitchell!
Add PKCS#7 and PKCS#12 to readme.
Change data representation of BigInteger

Store the integer parts of a BigInteger in an array instead of directly
in the BigInteger object. This improves performance of RSA key
generation on most browsers (Firefox, IE 10, Safari) by an order of
magnitude. Performance in Chrome stays more or less unchanged.
Merge pull request #19 from AchimO/master

Massive performance boost for RSA key generation in Firefox, IE 10, Safari.

Thanks @AchimO!
Update README.md
signedData initial support
Fixes for asn.1 structures w/undefined values.
Fix incorrect certificate in pkcs12 test file.