[Orgmode] Tag matches are now case-sensitive

[Carsten Dominik]

Dear all

I just pushed a change that makes tag searches from now on case- 
sensitive.
Making them case-insensitive was difficult to reconcile with an effort
to make the agenda faster, and something like Eric's org-collector *a  
lot*
faster.

I hope you have not relied on case-insensitivity of tag searches -  
please check
you custom agenda commands if this is an issue.

- Carsten