Support for customising the useragent

[eug48]

No description provided.

[waldyrious]

Hi @eug48, thanks for the PR! As a result of working in the svn-->git transition, I do have commit rights in this repo, but I haven't worked with mwclient development itself enough to feel confident about merging your commits right away. Could you add a brief explanation for the rationale of the changes? Thanks :)

[eug48]

Ok, sure @Waldir. When mwclient sends a request it includes a "Useragent" HTTP header that used to be set to "MwClient". However, this doesn't let the administrators of Wikipedia for example, identify the actual program that's making requests as all programs using mwclient look the same. According to the MediaWiki API docs (see http://www.mediawiki.org/wiki/API:Main_page#Identifying_your_client) they want to be able to identify each client individually.

[waldyrious]

Thanks. From what I can see, it looks good, I'll merge.