Remove config.h from public headers and include it in non-header files instead

I have an autotools-based project and want to use `libsrtp`. I have some symbol collisions such as `PACKAGE_NAME` macro.  This [link](http://inaugust.com/post/68) explains my problem well. And according to some [best practices](https://www.gentoo.org/doc/en/articles/autotools-practices.xml), `config.h` should not be included in the public header files.

What I did in this PR are
* Remove config.h from `aes.h` and `integers.h` which are indirectly referenced by public `srtp.h`
* I include `config.h` in every necessary non-header files to succeed the build(`make`) and test(`make runtest`)
* I include `config.h` at the top of `srtp_priv.h` instead.

Then my project includes some `AC_CHECK_HEADERS` in `configure.ac` to provide necessary macros like in `config.h` to make the public headers work.

If there is a better way to do it, please let me know. Thanks.