ixion 0.11.0

* C++11 is a hard requirement.

* implement R1C1 formula name resolver.

* remove boost dependency from the public headers.

* fix incorrect life-cycle management of pooled string instances.

* make it buildable on OSX.

* other general code cleanups.

* python

  * correctly catch and translate general_error into python's, for
    Document.append_sheet() method.

  * make python module build configurable.

  * add ixion.column_label() to convert numeric column indices into
    column labels.  A1 and R1C1 are supported.

ixion 0.9.1

* include python bindings test in the tarball.

* fix various portability problems in python bindings.

* several minor build fixes.
