Switch to gorilla websocket and btcrpcclient.

Closes #96.
Modify websocket endpoint from 'frontend' to 'ws'.
Return non-nil RPC results for 0 length slices.

If a JSON array result was successfully calculated, but the
slice/array is empty, the result must be marshaled as '[]' rather than
the JSON null value.  To do this in go, the RPC handlers should never
return nil slices for non-error returns, but return a non-nil slice
header with 0 length.

For example, an empty listtransactions result should be returned as
[]btcjson.ListTransactionsResult{}, rather than nil.
Randomize change transaction output index.

Based on a diff created by @dajohi.
Only use P2PKH outputs as sendfrom/many inputs.

Closes #89.
Improve tx creation performance.

Unspent inputs were being sorted multiple times in an inner loop.
This change moves the sort outside the loop so it is only performed
once.  While here, also move the transaction confirmation filter
outside the inner loop, as these can be calculated just once.
Fix unmined transaction resend logging.
Fix bug returning unconfirmed txstore records.

The incorrect slice was being appended to, causing some unconfirmedg
records to be dropped from the return result.
Mark finished rescan from notification.

Fixes #99.
No need to check map before removing a key.
Remove tx send/recv write order synchronization.

This was only necessary for a very old version of the transaction
store.  The current implementation stores both sent (debit) and
received (credit) records for individual transactions.
Remove unnecessary uint32 type conversions.
Remove unused goroutine.
Use an unbounded queue for waiting notifications.

Fixes a hang where a send on the notification chan can block due to
the queue being filled, and the current running notification making a
blocking call to the rpc client.

Fixes #100.
Improve txstore unspent output bookkeeping.

This change "reverses" the mapping used by the transaction store to
reference and lookup unspent credits.  Rather than mapping slice
indexes of a block, and then another block map for slice indexes of
transactions with unspent credits, and requiring a lookup through each
credit for whether it is spent or unspent, keep a simple map of
outpoints to a lookup key to find the transaction in a block.

This has a positive effect on performance when searching for previous
transaction outputs that have been spent by a newly-inserted
transaction.  Rather than iterating through every block with an
unspent credit, and then every transaction with unspent credits, a
simple map lookup can be done to check whether a transaction input's
previous outpoint is marked as unspent by wallet, and then access the
transaction record itself by the lookup key.  While transactions
created by wallet with the sendfrom/many RPCs may mark debits with the
previous credits already known, the previous outputs may still not be
known if a debiting transaction was added by rescan, or notified as a
result of a create+sendrawtransaction.
Pass txstore.Credit/Debits directly, not pointers.

The Credit and Debits structures are simple wrappers around an
embedded *txstore.TxRecord, as well as an output index in the case of
Credit.  This means that a Credit is at most two words, while a Debits
struct is just one.  To avoid the unnecessary garbage of creating
Credit and Debits structures on the heap (where the underlying
TxRecord likely already is), simply pass around everywhere as
non-pointer types, and modify the receivers for all Credit and Debits
methods to non-pointer receivers since none of them ever modify the
value.
Remove unneeded check of closed channel.
Fix various issues found by profiling.

This commit is the result of inspecting the results of both cpu and
memory profiling, to improve areas where wallet can be more efficient
on transaction inserts.

One problem that's very evident by profiling is how much waiting there
is for file (txstore, wallet) writes.  This commit does not attempt to
fix this yet, but focuses on the easier-to-fix memory allocation
issues which can slow down the rest of wallet due to excessive garbage
collection scanning.

While here, fix a race where a closure run as a goroutine was closing
over a range iterator.
Search unconfirmed txs when finding prev credits.

If a transaction is added that debits from previous transaction
outputs, and those outputs are still unconfirmed, it is possible that
if the credits were not already known (as is the case with
transactions notified after a sendrawtransaction), only mined unspent
transaction outputs would be searched and the unconfirmed unspent
credits would be missed.  This results in spent outputs still being
marked unspent.

This change fixes the above by also searching through unconfirmed
transactions when the previous credits must be lookup up, rather than
being pass from an AddDebits call.

Fixes issue #91.
Add basic transaction store logging.

The info log level (default) will produce output about confirmed and
unconfirmed transactions being inserted into the store, as well as
unconfirmed transactions which have been mined into blocks.  By
enabling the debug log level (-d TXST=debug), additional information
about transaction inputs and outputs is logged.  This includes the
total amount of previously-unspent outputs which have been marked
spent by the inserted transaction, and the output indexes and amounts
for each spendable output.  Additionally, the debug log level will log
whenever transactions are removed due to being a double spend of
another inserted transaction.