[FEATURE] Export history as a .csv

Add rpDownloadCsv attribute to rpDownload directive to specify that data
is downloaded as csv format.
[FEATURE] Export history as a .csv

Add button that triggers download of displayed History to CSV file.
[FEATURE] Export history as a .csv

Add exportCsv function and associated helper functions to extract the
displayed Transaction History into a string with a comma-delimited line
for each history entry.
[FEATURE] Export history as a .csv

Add getAmountSent function to calculate the amount sent if possible.
This was refactored from loadTx() in tx.js

In processTxn, add the following to returned transaction object:
- amountSent:  (together with currency if not XRP)
- sendMax: SendMax if it exists as an object (not if just String)
[FEATURE] Export history as a .csv

Move code within loadTx() that calculates amount sent to getAmountSent()
in jsonrewriter.js and invoke that instead.

NB the code that was moved to getAmountSent() has been changed to fix a
bug: the amount sent was not being calculated when the send and receive
currencies are different and one of them is XRP.
[FEATURE] Export history as a .csv

Correct for renamed function getAmountSent
Merge pull request #1439 from bss-/export-history-csv

[FEATURE] Export history as a .csv