prepare for travis-ci.org continuous integration

Travis CI is a free hosted continuous integration platform for
open-source projects.  It allows automated testing for GitHub-hosted
projects.

This commit adds a corresponding `.travis.yml` configuration file.
document.h: define __STDC_CONSTANT_MACROS

The C++ standard does not include the C99 macros used to set the (U)INT64
constants in document.h and reader.h (see adf66292 and ce1fece2).

Many implementations include their definition when the
__STDC_CONSTANT_MACROS preprocessor symbol is defined.

See e.g. http://www.cprogramdevelop.com/5272623/,
needed to successfully build in travis-ci.org's environment.
Merge pull request #24 from pah/fixes/travis-ci

Prepare travis-ci.org integration, fix build on Ubuntu 12.04 LTS