Fix for the "-Wterminate" warnings of GCC 6

[thrimbor]

This is just another small thing that came up when I started using cppcodec in my C++11 project.
There were a few methods which were defined with the noexcept-specifier but included a throw-statement. In C++11, when throwing an exception inside a noexcept-method, std::terminate() gets called instead of the usual exception handling. Usually this isn't the wanted behavior, which is why GCC 6 now spits out a warning message when doing that.
I resolved this by removing the noexcept-specifier from these methods. Since they were meant to get inlined and weren't introducing variables on the stack, this shouldn't affect performance in a negative way.

[jpetso]

Thanks. I'll just have to test if this has any effect on the generated binary size, but it should be a workable change. In the medium-term future, I hope to replace exceptions with error codes on the lowest level as mentioned in issue #4, which would make it possible to bring back the noexcept.

[jpetso]

Checked generated binaries and differences are negligible, in my GCC 6.1.1 binary of test_cppcodec even a kilobyte or so smaller in Release and MinSizeRel compared to the baseline. (I'm not sure why this would be the case, but oh well.) No significant impact on performance either.

Thanks again, your contribution is greatly appreciated :)