Use a better random seed.

time.clock() is CPU time, and so changes quite slowly, especially when
using a divisor of 100.  Change to .clock()%1000, and the seed changes
much more interestingly.
Refactor Selenium tests so I can write some more of them.
Make the scenarios more manipulable from tests.
A selenium test of problems.
Make create_xblock a method on WorkbenchRuntime
Make the test do a little bit more with the problems.
Use construct_block to get proper mixins, if any.
create_block is a better name.
Merge pull request #79 from edx/ned/fix-problems-again

Ned/fix problems again