Add tpmdata file io functions for keylime to read/write tpm data

[leonjia0112]

Added four functions for read/white/get/set tpmdata.json file where tpm data is stored in tpm.rs. Also added a tpmdata_test.json for testing purpose.

• Changed .travis.yml to single thread testing, because parallel testing causes the file changed by other tests and fail.
• Modified tpmdata read function with error handling without panic.

Compare to Lincoln lab version:
Original python version used a global variable tpmdata to store tpmdata in global scope. This implementation eliminated global variable is to store tpm data since it is not a frequent access variable.

Test passed. Ready for a review. Thanks.

[frozencemetery]

Please review and cleanup your diff.

[leonjia0112]

Test passed. Ready for another review. I changed Travis back to what it was, and now all the tests should pass without fail. Thanks.

[frozencemetery]

By "clean your diff", I intend "remove all changes which do not belong as part of this PR". Changes which are part of this PR should be only the ones required for fulfilling the goals stated in the commit message. (Incidentally, please fix spelling there.)

[leonjia0112]

Changed based on review:

1. Changed error from -1 to message that explains the detail of the error.
2. Changed Option to String
3. Used is_ok() in testing
4. Remove .unwrap() that panic the system with error handling and early return.

Test Passed. Ready for a review.
Thanks.

[frozencemetery]

Diff still not clean; see previous review.

[leonjia0112]

Now there are only tpm.rs and tpmdata_test.json files are changed in this PR.

Note:
tpmdata struct was removed because it is no longer used by the read_tpm_data() function.
File changed of main.rs and common.rs is because travis is complaining about the formatting, which it wasn't complaining before.

Test passed, ready for a review
Thanks

[leonjia0112]

Add a output_error_message() function for Err(Box::new(foramt!(...))) that can take a error message and a error and return a error message string.

.unwrap()s are removed with error handling Result<>.

Rebased onto the new PR.

Test passed. Ready for a review.
Thanks!

[frozencemetery]

@leonjia0112 I've made some changes. Please take a look and if you're satisfied they're okay, I'll merge.

[leonjia0112]

Those changes are decent. I am ok with those changes. Thanks :)