A new OID module has been created that contains the main OID searching
functionality based on type-dependent arrays. A base type is used to
contain the basic values (oid_descriptor_t) and that type is extended to
contain type specific information (like a pk_alg_t).

As a result the rsa sign and verify function prototypes have changed. They
now expect a md_type_t identifier instead of the removed RSA_SIG_XXX
defines.

All OID definitions have been moved to oid.h
All OID matching code is in the OID module.

The RSA PKCS#1 functions cleaned up as a result and adapted to use the
MD layer.

The SSL layer cleanup up as a result and adapted to use the MD layer.

The X509 parser cleaned up and matches OIDs in certificates with new
module and adapted to use the MD layer.

The X509 writer cleaned up and adapted to use the MD layer.

Apps and tests modified accordingly

 - Parsing of PEM files moved to separate module (Fixes ticket #13). Also possible to remove PEM support for systems only using DER encoding
 - Parsing PEM private keys encrypted with DES and AES are now supported (Fixes ticket #5)
 - Added tests for encrypted keyfiles

 - Fixed bug resulting in failure to send the last certificate in the chain in ssl_write_certificate() and ssl_write_certificate_request()