addresses #69

• Adds getex command
• Adds GetEx method to generic API
• Adds new struct GetExOptions
• Minor fix to handleExpire and handleExpireAt functions
  • these would always assign and reassign the variable expireAt unnecessarily in the case of pexpire and pexpireat
• Fixed inline doc for generic API Expire method to correctly reflect that the milliseconds argument is milliseconds and not seconds
• Fixed issue in GetRandom method of SortedSet.
  • I was getting test failures for Test_SortedSet/Test_HandleZRANDMEMBER about 70% out of 10-15 runs. I was able to observe that GetRandom would sometimes return an empty value in one of it's indexes. I narrowed this down to how DeleteFunc operates. I implemented manual index swapping instead, and no longer saw any failures here (checked with about 20 runs).

While testing I observed some strange behavior with test-race. I had two modules fail during test-race, acl and echovault, each on separate runs. There wasn't any sort of traceback provided with either which was strange. For each one, I then attempted to run the tests for that module independently and all tests passed. After running test-race again after each of these failures I received no more failures. However, I tried clearing the testcache and when running make test I started receiving failures on Test_SortedSet/Test_HandleZRANDMEMBER so I debugged that. The other failures I saw no longer popped up.