Fixing uses of AutoPtr in MOOSE for upcoming libmesh refactoring.

This patch does not require a libmesh update to work, it only enforces
proper AutoPtr usage.

Refs #3798.
It is ambiguous to construct a std::unique_ptr with an explicit (NULL) argument.

This is apparently because there is a 'nullptr' constructor for this
class.  Luckily, using the default constructor instead works just fine.

Refs #3799.
Fixing warnings in solid mechanics module.

@backmari, can you comment on why you compute all these values but
they aren't used for anything?

Refs #1777.
Merge pull request #3820 from jwpeterson/fix_warnings

Fixing warnings in solid mechanics module.
Merge commit '27ac890d617d027c50c04ea02db4bea91dffa018'
Merge pull request #3799 from jwpeterson/fix_autoptr

Fixing uses of AutoPtr in MOOSE for upcoming libmesh refactoring.
Merge commit 'c3fcd77e3ac763bc61625d77316dc34148f8bbf0'