Addtional docstrings in the differential geometry module.

Doctests were slightly updated. Explicit mention about the effect of different
coordinate system choices was added.
Work on itegral curves in the differential geometry module.

Two new functions were added to the differential geometry module that deal with
integral curves. Tests for these functions are supplied.

Integral curves are inherently represented in coordinate dependent way because
there is no coordinate independent way to present movement from point to point
on a general manifold (i.e. there is no such thing as difference between two
points).

The first function is `intcurve_series`. It produces the series expantion of an
integral curve when given the starting point and the coordinate system in which
it must work.

Specify a starting point and a vector field:
>>> start_point = R2_r.point([x, y])
>>> vector_field = R2_r.d_dx

Calculate the series:
>>> series = intcurve_series(vector_field, t, start_point, n=3)
>>> series[0]
[x]
[y]
>>> series[1]
[t]
[0]
>>> series[2]
[0]
[0]

The second function is `intcurve_diffequ` that works in the same way as the
first one, however it returns a system of differential equations describing the
integral curve.

Specify a starting point and a vector field:
>>> start_point = R2_r.point([1, 0])
>>> vector_field = -R2.y*R2.d_dx + R2.x*R2.d_dy

Get the equation:
>>> equations, init_cond = intcurve_diffequ(vector_field, t, start_point)
>>> equations
[f_1(t) + Derivative(f_0(t), t), -f_0(t) + Derivative(f_1(t), t)]
>>> init_cond
[f_0(0) - 1, f_1(0)]
solve returns RootOf objects for polys of high degree.

In [1]: solve(x**5+x**4+x**2+4)
Out[1]:
[
      ⎛ 5    4    2       ⎞
RootOf⎝x  + x  + x  + 4, 0⎠,

      ⎛ 5    4    2       ⎞
RootOf⎝x  + x  + x  + 4, 1⎠,

      ⎛ 5    4    2       ⎞
RootOf⎝x  + x  + x  + 4, 2⎠,

      ⎛ 5    4    2       ⎞
RootOf⎝x  + x  + x  + 4, 3⎠,

      ⎛ 5    4    2       ⎞
RootOf⎝x  + x  + x  + 4, 4⎠]
Merge pull request #1358 from Krastanov/solve_roots

solve returns RootOf objects for polys of high degree.
	Basic groups, orbit calculations, direct product, minor fixes

	Fixed the issues pointed out in the pull request discussion.

	Implemented constructors for symmetric, cyclic, dihedral and
	alternating groups as permutation groups, setting their
	basic properties.

	Replaced some of the orbit/stabilizer functionality present -
	the functions orbit, stabilizer, orbit_transversal - with faster
	implementations. Implemented Schreier vectors and
	group elements sending a given element to another element in
	its orbit.

	Implemented direct products for permutation groups.

	Made minor fixes in the docstrings and removed some duplicate
	functionality from permutations.py.
Minor spelling fix.
Merge pull request #1316 from Krastanov/vector_analysis_2

Vector analysis 2
Merge pull request #1319 from amakelov/week1

Basic groups, orbit calculations, direct product, minor fixes
Use sort_key in FiniteSet's element_sort_fn.

This will make FiniteSet less dependent on the machine.
Fix the ordering of elements in the doctests in sympy.categories.

After the change to the ordering function of FiniteSet, the order of
elements of sets printed in doctests has also changed.
Merge pull request #1362 from scolobb/finiteset-sort_key

Use `sort_key()` to sort those elements in `FiniteSet` which have it.
Add Travis CI script

Currently the script tests the *installed* sympy (that is, after "python
setup.py install" is run), for Python 2.5, 2.6, 2.7 and 3.2.
Install sympy.categories in setup.py

This fixes a few test failures when testing the installed SymPy.
Merge pull request #1363 from certik/travis-ci

Add Travis CI integration
README: add links to Travis CI and sympy-bot
Merge pull request #1365 from certik/travis-ci

README: add links to Travis CI and sympy-bot