make polyroots more robust

* fix tolerance calculation

We now set the tolerance `tol` directly equal to `ctx.eps`. This is the right
way to do it, since if more working precision is needed, the user needs to
assign the `extraprec` argument.

* use `extraprec` argument

Previously this argument was not used at all by a mistake and instead the
default value 10 was assigned directly.

* chop small numbers in "cleanup" mode

Previously only small real or imaginary parts were chopped. But if abs(x) is
close to zero, we want to chop this case as well.

* Raise NoConvergence error if convergence is not achieved

* always refine all roots

Previously if the given root was already converged, it wasn't refined. But this
seems to increase the number of iterations needed to converge considerably.
Don't use py.magic to figure out rootdir
Enable Travis-CI tests, closes #16
Merge pull request #17 from skirpichev/enable-travis

Enable travis-ci builds
Add travis banner in the README.rst
Merge pull request #18 from skirpichev/travis-banner

Add travis banner in the README.rst
Add a test for polyroots
Add a test for NoConvergence
Use +ctx.eps instead of ctx.eps*1
Use extraprec context manager

Instead of try/finally blocks.
Use workdps context manager in the test
polyroots: Improve documentation

We did few changes here and there and added a new section about how the
precision works.
Merge pull request #15 from certik/polyroots

Make polyroots more robust
polyroots: clarify a docstring about extraprec
docs: add a missing word
Merge pull request #19 from certik/docs

polyroots: clarify a docstring about extraprec
Fix line endings handling in git

See [1] for more information.

[1] https://help.github.com/articles/dealing-with-line-endings
Normalize line endings
Merge pull request #20 from certik/line-endings

Fix line endings
Update a couple of URLs in README.rst