Added ShortCode to Organisation

ShortCode has been available in the API since the Mar 11, 
2013 version 2.22 release so have put it in the wrapper.  
The caveat described in https://community.xero.com/developer/discussion/1167775 
"ShortCodes will be changing in an upcoming release on March 
25th 2013 - after this date, the ShortCode retrieved via 
the API will not change, but prior to this, will be unique, 
but the value is not permanent" has passed so this should be safe. 
Merge pull request #30 from TDaphneB/master

Added ShortCode to Organisation
Remove usage of Attachment.Filename property (obsolete)
Version bump to v1.1.0.29
Added RemainingCredit property to credit note

The remaining credit property is returned by the API but was not in the credit note class.
Merge pull request #31 from TDaphneB/master

Added RemainingCredit property to credit note class
Bump version to v1.1.0.30

Added RemainingCredit property to credit note class
Upgrade nuget.exe to v2.5.0 (it now published to nuget.org)
Add better exception handling for the "could not create ssl/tls secure channel" exception, that tells the user exactly which certificates need to be checked.

Signed-off-by: Eddie Loeffen <eddieloeffen@gmail.com>
Add a test for the new exception handling

Signed-off-by: Eddie Loeffen <eddieloeffen@gmail.com>
Merge pull request #32 from Vinsight/master

Better exception message around permissions on client SSL certificates
Bump version to v1.1.0.31
Merge https://github.com/XeroAPI/XeroAPI.Net