Added invoice export function and started implementation of unitofwork pattern (#43)

* complete read invoices from db

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* updated nuget package spapi

* WIP

* wip, now test

* wip, jut need to fix tax inclusive line amounts not supported

* wip

* wip, before I f everything up

* no, it complies now, this is the one before I f everything up

* wip

* wip

* wip, logic ready for testing

* wip it builds!!!!

* wip tested, working, need to complete the gui section

* wip

* wip

* wip - created export invoice data delete, time for testing

* wip testing phase

* wip - delete function fully tested and working

* wip on to sorting out the issue with settlement invoices not tallying

* wip

* wip

* wip

* wip

* wip before I complete change the ReadInvoiceLineItem sections

* that appears to have worked, on with the main quest

* no it's doesn't work, saving before i remove the confusing cache system (just use a dictionary!!)

* wipping picadilli

* wip

* wip

* implemented uow on inovice export, now for testing

* wip

* wip all tested do invoice currency convertion fearure

* wip

* pretty much done so long as xero accepts the exported invoices

* Complete!

src/bnhtrade.Core/Data/Database/Connection.cs

@@ -11,11 +11,11 @@ namespace bnhtrade.Core.Data.Database
     public class Connection
     {
         //protected readonly string SqlConnectionString;
-        private Model.Credentials.bnhtradeDB dbCredentials;
+        private Model.Credentials.bnhtradeDB _dbCredentials;
 
         protected string SqlConnectionString 
         { 
-            get { return dbCredentials.ConnectionString; }
+            get { return _dbCredentials.ConnectionString; }
         }
 
         public Connection()
@@ -44,7 +44,7 @@ namespace bnhtrade.Core.Data.Database
                 }
 
                 var dbCredentials = new bnhtrade.Core.Model.Credentials.bnhtradeDB(dataSource, userId, pass);
-                this.dbCredentials = dbCredentials;
+                this._dbCredentials = dbCredentials;
             }
             catch (Exception ex)
             {
@@ -76,7 +76,7 @@ namespace bnhtrade.Core.Data.Database
                 }
 
                 var dbCredentials = new bnhtrade.Core.Model.Credentials.bnhtradeDB(dataSource, userId, pass);
-                this.dbCredentials = dbCredentials;
+                this._dbCredentials = dbCredentials;
             }
             catch(Exception ex)
             {
@@ -91,7 +91,7 @@ namespace bnhtrade.Core.Data.Database
             {
                 throw new Exception("DB credentials object is null");
             }
-            this.dbCredentials = dbCredentials;
+            this._dbCredentials = dbCredentials;
         }
     }
 }