Created procedure Blockly.Python.provideFunction_().
This improves abstraction and makes it easier to add our own procedures
to generated code.  It enables an unlikely bug if different parts of our
code generate procedures with the same desired name but different bodies.

Fixed minor bugs:
- isPrime was missing required import.
- SQRT1_2 used 1/2 instead of 1.0/2.

Perhaps make these functions follow the style guidelines in a future CL.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/10064043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1034 c400ca83-b69d-9dd7-9705-49c6b8615e23
Picky language fixes.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/10098043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1035 c400ca83-b69d-9dd7-9705-49c6b8615e23
Adds "by" (step) argument to "count from" block.

Splits control.xml into control[123].xml.  I'll remove control.xml
in a separate CL since I don't know the effect of "svn rm/mv" after "svn cp".

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/9839043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1037 c400ca83-b69d-9dd7-9705-49c6b8615e23