doc: add wraparound module to documentation

Closes #113.

doc/source/reference/index.rst

@@ -38,3 +38,4 @@ The following modules provide functionality for special applications.
    kwant.digest
    kwant.rmt
    kwant.kpm
+   kwant.wraparound

doc/source/reference/kwant.plotter.rst

@@ -12,6 +12,7 @@ Plotting routine
    plot
    map
    bands
+   spectrum
 
 Data-generating functions
 -------------------------

doc/source/reference/kwant.wraparound.rst

+:mod:`kwant.wraparound` -- Wrapping around translational symmetries
+===================================================================
+.. module:: kwant.wraparound
+.. autosummary::
+   :toctree: generated/
+
+   wraparound
+   plot_2d_bands

kwant/wraparound.py

@@ -384,7 +384,7 @@ def plot_2d_bands(syst, k_x=31, k_y=31, params=None,
 
     See Also
     --------
-    `kwant.plotter.spectrum`
+    kwant.plotter.spectrum
     """
     if not hasattr(syst, '_wrapped_symmetry'):
         raise TypeError("Expecting a system that was produced by "