Bootstrap setup.py: resolve dependency to numpy

With setuptools the `setup_requires` parameter can be used to install
missing dependencies before executing a build command. By subclassing
the build_ext command, the numpy headers can be added dynamically when
the numpy dependency is already installed.
Remove requirements.txt file

The file was previously used to install missing build time dependencies.
This has now become unnecessary:

- Cython is not a build-time dependency for the source distribution
  anymore (see 4bd9db9)
- With the previous commit, numpy will be installed during setup before
  the build_ext command needs it.
Simplify include/library path discovery

Note that this introduces a *minor* incompatibility when invoking
setup.py: The '--madxdir' argument can be given only once. Passing it
multiple times is now considered an error (which one should we take?).

If library and headers reside in different locations, you should use

    python setup.py build_ext -L<library-dirs> -I<include-dirs>

to specify them manually.
Remove automatic MAD-X discovery from setup

setuptools should be able to automatically find and use a system wide
installation. For custom installations, '--madxdir=...' can always be
used.
Update changelog
Fix new .travis.yml file and clarify wording
Fix the numpy import problem
Merge branch 'master' into bootstrap-setup

Conflicts:
	CHANGES.rst
	setup.py
Use custom --madxdir for builds on test servers
Merge remote-tracking branch 'origin/master' into bootstrap-setup
Merge remote-tracking branch 'origin/master' into bootstrap-setup
should now be version 0.8
Add workaround for python3 bug on initial build
Allow passing multiple --madxdir parameters

The directories are just accumulated and all added to the path
variables.
Merge pull request #46 from pymad/bootstrap-setup

Use setup_requires for numpy