Quick and dirty patch for getting transactions from a category

[rob-carruthers]

Hi, didn't think it was worth branching just for this. Very quick patch for another bit of functionality, just made it by poking around. See what you think. The methods of Transaction work too. Thanks

diff --git a/ynab/models.py b/ynab/models.py
index 1e0b3b4..4f9bf53 100644
--- a/ynab/models.py
+++ b/ynab/models.py
@@ -182,6 +182,10 @@ class Category(CategoryModel):
     def full_name(self):
         return '{}/{}'.format(self.master_category.name, self.name)

+    @property
+    def transactions(self):
+        return self._ynab.transactions.filter('category', self)
+

 class MasterCategory(CategoryModel):
     _entity_type = schema.MasterCategory

[aldanor]

@grooveharder Good point, I must have missed this one! Added in 37e7982.