trac #12630 Add representations of quivers and quiver algebras
Implementation of quivers as associative magmas

* Fix most import statements from doctests of quivers/representation.py
* add QuiverRepElement to all.py
* fix some doctest errors
#12630: Use existing Sage infrastructure for quiver representations and their homspaces and morphisms
Merge branch 'ticket/15623' into ticket/12630

* ticket/15623: (760 commits)
  Re-insert a bit of code that had been erroneously deleted in the previous merge commit
  Updated Sage version to 6.1.rc0
  only use CONFVERSION after it has been incremented
  note that the E6->A5 rule is now implemented
  document the glibc bug workaround patch
  Avoid scanf instead of just ignoring rest of file
  bump confball version
  fix for the m4_divert_push error
  sage-clone-source: allow downloading the configure tarball
  Require autoconf 2.64
  revised thematic tutorial regarding maximal_subgroup(s) method
  reference: branching_rules: minor fixup
  bump confball version
  removed bogus commits
  LinearMatroid.is_field_equivalent now tests if self is other before calling LinearMatroid._is_field_isomorphism.
  implemented maximal_subgroup(s) as a WeylCharacterRing method etc.
  Updated Sage version to 6.1.beta6
  Do not use the GNU extension chmod --recursive
  Workaround for glibc-2.18 scanf bug when building ATLAS
  Added  free_group and finitely_presented_group
  ...
Merge branch 'ticket/15810' into ticket/12630, to make acyclicity tests work for immutable directed graphs

* ticket/15810: (186 commits)
  trac #15810: The graph data structures may have a ._directed but no .directed
  trac #15810: Immutable directed graphs should know that they are directed
  Updated Sage version to 6.2.beta1
  Use posix locale for regex
  Let ncurses build again on Solaris.
  Tweaks for speed in sparse graph backend for #15715.
  Updated Sage version to 6.1.1
  trac #14508: better now?
  Trac 14508: bugfix in jordan_form
  Update README.txt re installation with OS X Mavericks
  Fix test for empty string in FLINT build system.
  Update to version 0.10.8.2
  Update sagenb to 0.10.8.1
  Replace two more "except StandardError:"
  Updated Sage version to 6.2.beta0
  Fix documentation syntax
  GSL FFT: indentation (& related) fixes in docstrings
  Minor documentation fixes.
  #Documentation for gsl/fft.pyx
  #10058: Segfault in backward and inverse FFT for 2**n elements
  ...
Do not introduce a separate class for quivers (use immutable DiGraph)
Do not use "invalid path" for quivers. Fix all doctests.
mostly grammatical fixes to documentation; not a review at all
Merge branch 'develop' into quivers
update doctests
allowed empty quiver; made homogeneous components work for quivers with cycles; fixed various places in doc
Some more doc fixes
a few more doc improvements
simplifying all_paths and improving more doc
Trac 12630: Fix QuiverRepHom by renaming __call__ to _call_
Trac 12630: Fix coercion for path algebras
Trac 12630: Monomials of path algebras must belong to the underlying quiver
minor optimization, one() twice as fast
minor edits to doc again
fixes, mostly decorative