Added AuxKernel for extracting component from variable gradient
(closes #3881)
Added test for new AuxKernel
(refs #3881)
Merge pull request #3889 from aeslaughter/grad-component-3881

Added AuxKernel for extracting component from variable gradient
Merge commit 'f00b4f450e96c77c4b42365ba67ffb0286e6c0cd'