Merge refactor-algebra. Resolves #45. Resolves #49.

Large merge of a refactorization of the algebraic
components of abelfunctions. Summary of changes:

* Introduces PuiseuxXSeries and PuiseuxTSeries objects: a
  "manual" implementation of puiseux series for performance
  and stability purposes.
* An update and minor performance improvement of the
  `integral_basis()` function. Work still needs to be
  done on this.
* An update of `singularities()` and `differentials()` to
  use the new puiseux series objets.
* Documentation for these new objects and functions. Adds
  `sphinx_rtd_theme` as a subtree to the repository.