[README] markdown + build status

 * Using markdown extension, added `README` symlink
 * Added travis build status badge
[pip+travis] use `requirements.txt`

Use `requirements.txt` instead of setuptools' `extras_require`
for installing basemap.

That is because PyPi basemap version won't find libgeos library
so we resolve to use latest basemap from git. `Extras_require` don't
allow providing custom links, only PyPi package names, so we have to
specify links in requirements.txt. `dependency_links` argument to
`setup` call is meant for cruicial dependencies, not custom ones, so we
don't use them neither.