Add symbol to currency code mapping

[ddormer]

During cldr import the currency_codes mapping is created on data and
internally exposed by the Locale.currency_codes property.
The numbers.get_currency_code API is made available to convert a
currency symbol into a currency code.

Partially addresses issue #141

[akx]

I wonder if it's worth it to actually store the inverse mapping in the .dat file, since it's exactly the same data as currency_symbols, but keys and values reversed.

I think we should just synthesize currency_codes when first accessed within the Locale object.

[ericzolf]

I personally use the following workaround in the meantime:

import babel.numbers as babelnum

CURRENCY_MAP = { babelnum.get_currency_symbol(x): x
                 for x in babelnum.list_currencies()
                 if x != babelnum.get_currency_symbol(x) }

def get_currency_code(currency_symbol):
    return CURRENCY_MAP.get(currency_symbol, currency_symbol)