Add asan to travis

Hi Karl,

I've added an additional build to the travis configuration for clang. The motivation is that the default clang installed on the travis build machines is recent enough to support the address sanitizer library. I added an option to the build system to build with address sanitizer enabled and that's used on travis now.

I have seen intermittent build failures with the clang builds (the gcc builds are fine). Perhaps you have an idea whether those are false positives or what. All tests pass for me locally with clang 3.5 (llvm/trunk 202722) with address sanitizer enabled.

Btw it is possible to test with multiple versions of gcc. See discussion here https://github.com/travis-ci/travis-ci/issues/979 and the following .travis.yml configuration for an example: https://github.com/datagraph/libsparql/blob/master/.travis.yml. I have never set this up myself but I could give it a shot. Would that be of interest?

Cheers,
Dominic