Merge pull request #534 from akhileshdarjee/master

[Fix]SMS Center  & Purchase Register
[fix] invoice with zero values
Merge branch 'master' of github.com:webnotes/erpnext
[Trends Teports] cleanup
Merge pull request #537 from saurabh6790/master

[Trend Reports] Cleanup
[fix] cleanup data
Merge branch 'master' of github.com:webnotes/erpnext
[fixes] ignore single doctypes in chekcing linked documents
[Fix] Total tax in Purchase register