Parse json floats as string

With python-2.7 the float values are sometimes rounded to unacceptable
levels, e.g. stripping the last two digits for values over 100k BTC.
This change parses floats as strings to avoid rounding.

Refactored get_url out of fetch_json to make it easier to add
new tx_api with a different url scheme.

trezorlib/tx_api.py

@@ -32,19 +32,23 @@ class TxApi(object):
         self.network = network
         self.url = url
 
+    def get_url(self, resource, resourceid):
+        url = '%s%s/%s' % (self.url, resource, resourceid)
+        return url
+
     def fetch_json(self, resource, resourceid):
         global cache_dir
         if cache_dir:
             cache_file = '%s/%s_%s_%s.json' % (cache_dir, self.network, resource, resourceid)
             try:  # looking into cache first
-                j = json.load(open(cache_file))
+                j = json.load(open(cache_file), parse_float=str)
                 return j
             except:
                 pass
         try:
-            url = '%s%s/%s' % (self.url, resource, resourceid)
+            url = self.get_url(resource, resourceid)
             r = requests.get(url, headers={'User-agent': 'Mozilla/5.0'})
-            j = r.json()
+            j = r.json(parse_float=str)
         except:
             raise RuntimeError('URL error: %s' % url)
         if cache_dir and cache_file:
@@ -88,7 +92,7 @@ class TxApiInsight(TxApi):
 
         for vout in data['vout']:
             o = t.bin_outputs.add()
-            o.amount = int(Decimal(str(vout['value'])) * 100000000)
+            o.amount = int(Decimal(vout['value']) * 100000000)
             o.script_pubkey = binascii.unhexlify(vout['scriptPubKey']['hex'])
 
         if self.zcash: