[MITx:6.00.1x] Add solutions to problem set 3
[MITx:6.00.1x] Add solutions to problem set 4
[MITx:6.00.1x] Add slides from lectures 9-10
[MITx:6.00.1x] Add slides from lectures 11-12
[MITx:6.00.1x] Add exercises from lecture 11
[MITx:6.00.1x] Add exercises from lecture 12