Minor updates in dof handler.
- getters marked const
- virtual methods marked override
- mesh renamed to mesh_
- added getters for mesh and finite elements
FieldFE - class for evaluation of function given by finite element approximation.
Unit test for FieldFE. Currently tests P1 and RT0 finite elements in 2D.
Fix build under Cygwin.

- add to_string method.
Force usage of cmake 2.8.8 and higher.

- we need it for Object libraries support in unit_test
Merge branch 'JS_field_fe'