added a sample for lib usage

tests/sample.c

@@ -0,0 +1,69 @@
+#include <pcp.h>
+
+int main() {
+  Buffer *inbuf;
+  pcp_key_t *alice, *bob;
+  pcp_pubkey_t *alicepub, *bobpub, *pubhash;
+  Pcpstream *clear_in, *crypt_out, *clear_out;
+  char message[] = "hello world"; 
+
+  /* generate the keypairs for both */
+  alice = pcpkey_new();
+  bob = pcpkey_new();
+
+  /* get the public parts of them */
+  alicepub = pcpkey_pub_from_secret(alice);
+  bobpub = pcpkey_pub_from_secret(bob);
+
+  /* put the clear text message into the stream */
+  inbuf = buffer_new(32, "a");
+  buffer_add_str(inbuf, message);
+  clear_in = ps_new_inbuffer(inbuf);
+
+  /* create the output stream as buffer */
+  crypt_out = ps_new_outbuffer();
+
+  /* prepare the pubkey recipient list (only 1 recipient: Bob) */
+  pubhash = NULL;
+  strncpy(bobpub->id, pcp_getkeyid(bobpub), 17);
+  HASH_ADD_STR( pubhash, id, bobpub);
+
+  /* actually encrypt the message, don't sign it
+     Alice is the sender, Bob is the recipient */
+  pcp_encrypt_stream(clear_in, crypt_out, alice, pubhash, 0);
+
+  /* now, print the encrypted result */
+  fprintf(stderr, "Alice encrypted %d bytes for Bob:\n", strlen(message));
+  buffer_dump(ps_buffer(crypt_out));
+
+  /* ---- encryption don, now decrypt ---- */
+
+  /* prepare the output buffer stream */
+  clear_out =  ps_new_outbuffer();
+
+  /* in order for the decryptor find the senders public key,
+     we need to put it into the global hash. this step can be
+  omitted when using a Vault. */
+  pcppubkey_hash = NULL;
+  HASH_ADD_STR( pcppubkey_hash , id, alicepub);
+
+  /* try to decrypt the message */
+  if(pcp_decrypt_stream(crypt_out, clear_out, bob, NULL, 0) == 0)
+    fatals_ifany();
+  else {
+    /* and finally print out the decrypted message */
+    fprintf(stderr, "Bob decrypted %d bytes from Alice:\n", buffer_size(ps_buffer(crypt_out)));
+    printf("Decrypted message: %s\n", buffer_get_str(ps_buffer(clear_out)));
+  }
+
+  ps_close(clear_in);
+  ps_close(crypt_out);
+  ps_close(clear_out);
+
+  free(alice);
+  free(alicepub);
+  free(bob);
+  free(bobpub);
+
+  return 0;
+}