Proposing a PR to fix a few small typos

# Issue Type

[x] Bug (Typo)

# Steps to Replicate and Expected Behaviour

- Examine restkit/resource.py and observe `parameterss`, however expect to see `parameters`.
- Examine restkit/resource.py and observe `optionnal`, however expect to see `optional`.
- Examine restkit/resource.py and observe `additionnal`, however expect to see `additional`.
- Examine restkit/filters.py and observe `agains`, however expect to see `against`.
- Examine restkit/filters.py and observe `wicth`, however expect to see `which`.
- Examine restkit/client.py and observe `unkown`, however expect to see `unknown`.
- Examine restkit/__init__.py and observe `tupple`, however expect to see `tuple`.
- Examine doc/sitemap_gen.py and observe `suppressable`, however expect to see `suppressible`.
- Examine doc/pool.rst and observe `sesssion`, however expect to see `session`.
- Examine restkit/client.py and observe `immediatly`, however expect to see `immediately`.
- Examine restkit/errors.py and observe `happend`, however expect to see `happened`.
- Examine restkit/wrappers.py and observe `appplication`, however expect to see `application`.

# Notes

Semi-automated issue generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

To avoid wasting CI processing resources a branch with the fix has been
prepared but a pull request has not yet been created. A pull request fixing
the issue can be prepared from the link below, feel free to create it or
request @timgates42 create the PR. Alternatively if the fix is undesired please
close the issue with a small comment about the reasoning.

https://github.com/timgates42/restkit/pull/new/bugfix_typos

Thanks.