Travis support

[piponazo]

Adds travis support for building the project on Linux and Mac OSX with different configurations. It actually helped me to find some issues in the CMake code :)

[clanmills]

Thanks very much for doing this, Luis. I hope you've had a nice weekend.

This hasn't solved the issue with libintl on MacOSX. However, I can see you're been changing stuff concerning linking exiv2 with other libraries. I've never investigated the .po files Thomas (our German Contributor), knows about them. For sure we initialise libintl from exiv2.cpp. I'm not sure the .po files are used by the library. If they are not, libintl shouldn't be linked to libexiv2. I'll email Thomas and ask him about this.

[clanmills]

Whoops, I spoke too soon. I've pulled down a fresh copy of the code from GitHub and it builds. Thanks for fixing this. And I'm delighted to see that CMake build is publishing the correct headers in /usr/local/include/exiv2.

[piponazo]

You are welcome :)

Right I did not check that yet, but now I can check it out even at home using travis.

The problem is that we are assuming the existence of Intl:

if( EXIV2_ENABLE_NLS )
    find_package(Intl)
    if(Intl_FOUND)

But I will experiment this week with that. It should be easily fixable ;)

[clanmills]

This seems totally splendid. Is there a Travis node actually building the code?

[piponazo]

Yes, but only on my fork:
https://travis-ci.org/piponazo/exiv2

I left you some messages on slack explaining the alternatives we have to enable the support in the main repo