build: Add travis continuous integration support

This enables the use of travis-ci via github. Linking any ovs
github repo to travis-ci [0] will automatically lead to a build
and testsuite run being triggered for each new commit or pull
requests against the repo.

The introduction of any warnings will cause the build to fail as
both gcc and clang are invoked with -Werror.

The build matrix currently includes:
  * --disable-ssl
  * --with-linux=linux-3.14.7
  * --with-dpdk=1.7.0

Each of these builds is performed with gcc+sparse and clang. The
testsuite is only run for the --with-linux build to keep the total
build time on an acceptable level.

Build failures are reported to the author and commiter by email,
through the travis-ci web UI, and in the github pull request.

A full build report example can be found here [1]

[0] http://travis-ci.org/
[1] https://travis-ci.org/tgraf/ovs/builds/33485228

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>