Unrefs the connection even if not in pool when removing in EvdConnectionPool

This was causing the connection to leak memory.
Makes Javascript web transport and JSON-RPC libraries work with require.js

If define.amd is defined, the libraries will produce anonymous modules.
Adds method isClosed() to Evd.Peer in Javascript Web transport

Also improves peer closing logic.
Removes checks for context being an Evd.Peer

in _transportWrite() of Javascript Evd.Jsonrpc. This is a workaround to the
fact that when using require.js AMD, peer.prototype and local Evd.Peer are
not referentially equal. Seems like a bug in require.js, confirmation needed.
Makes tests fail with warnings (-Werror) and removes all warnings from tests
Raises required version of GnuTLS to 3.0.0

This is to start a slow transition away from libgcrypt, and benefit from the
new higher level API in GnuTLS 3.0.0.
Removes 'enc-priv/dec-pub' test case from PKI tests
Removes methods decrypt() and decrypt_finish() from EvdPkiPubkey
Removes unused dispose() method from EvdPkiPubkey
Removes methods encrypt() and encrypt_finish() from EvdPkiPrivkey
Removes unused dispose() method from EvdPkiPrivkey
Completes EvdPkiKeyType enum with remaining values from gnutls_pk_algorithm_t
Adds new function evd_error_propagate_gnutls()

If @gnutls_error_code argument represents a GnuTLS error, the provide GError
with be filled with the corresponding error.
Migrates EvdPkiPrivkey from gcrypt to GnuTLS
Migrates EvdPkiPubkey from gcrypt to GnuTLS
Adds new methods verify() and verify_finish() to EvdPkiPubkey

This API asynchronously verifies that a given signature is valid for
the given data. By now, only SHA256 hashed signatures are supported.
Adds new methods sign() and sign_finish() to EvdPkiPrivkey

This API asynchronously signs the given data (currently using SHA256 hashing
algorithm only).
Adds new methods generate() and generate_finish() to EvdPkiPrivkey

This API asynchronously generates a PKI private key of the given type
(RSA, DSA, etc).
Adds new method get_public_key() to EvdPkiPrivkey

This API returns an EvdPkiPubkey object with public key material extracted from
this private key.
Updates PKI test to include new API