Relicensed to MIT license

The reasons for the relicensing are two: profiting from the
higher popularity of MIT license over...

Make numba optional, fix coverage numbers

Now if poliastro.test(coverage=True) is called, poliastro.jit will be monkeypatched
to use a dumm...

Fixes parabolic orbits, closes #37

The code is not complete yet as the rest of the orbits of the planets
have to be plotted too.
Now...

In this step the notebooks were converted to the 4.0 format.

Updated for numba 0.18, closes #27

This failure means the Stumpff functions need tests too.

Now exceptions with messages can be raised and the math.gamma
function is supported.

Added AppVeyor configuration, closes #3

Finally managed to produce a proper coverage report, tuning
.travis.yml conveniently and adding a...

Removed all Fortran sources and updated README accordingly.

Travis CI doesn't have X server.

I cannot use setuptools. Dammit.

Also fixed epoch of these, which was not being assigned correctly.

Basically rv2coe is ill-conditioned for equatorial and/or circular
orbits, which have low or zero...

This way I avoid all circular imports at the cost of losing one
handy method. Now this:

>>> from...

If I want to provide a method on `State` to plot, I think I have
to slightly break the consistenc...

Now the plane of the first plotted state is set as the plotting
plane for the rest of the states....

Both in hohmann and bielliptic the magnitude of the impulses was
correct but not their direction....

Now the extension modules are placed in the correct directory.

This helps splitting functions and organizing the plotting module,
already imported but yet to be...