restructure certs/keys; add key/cert generation

NID is also now an exposed type
improve GC handling (fixes #10)
Merge pull request #18 from lunixbochs/keygen

restructure certs/keys; add key/cert generation
add pem.go with SplitPEM function
add support for chains in ListenAndServeTLS
add NewCert..Store(), LoadCert..sFromPEM()
Merge pull request #21 from lunixbochs/store

Add support for loading chains from a single PEM