Do not convert numeric types to string

FIX: #149
Merge pull request #153 from bmihelac/149-excel-export-and-numbers

Do not convert numeric types to string
Update changelog.rst
Preparing release 0.2.5
Back to development: 0.2.6
Merge branch 'master' of github.com:bmihelac/django-import-export
remove invalid form action "form_url"
Merge pull request #154 from brad/invalid_variable

remove invalid form action "form_url"
fixed #105 unicode import
Merge pull request #155 from GroSte/master

fixed #105 unicode import
Update AUTHORS
Try to use OrderedDict instead of SortedDict, which is deprecated in Django 1.7. Fall back on SortedDict if we are aren't on Python 2.7 or higher.
Merge pull request #157 from w0rp/master

Use OrderedDict where available instead
Update AUTHORS
Added use of get_diff_headers method into import.html template
Merge pull request #158 from nastyako/master

Added use of get_diff_headers method into import.html template
Update AUTHORS
Update changelog
Preparing release 0.2.6
Back to development: 0.2.7