[#51] Rename Page.edit() to Page.text()

since the method does not edit the page, but returns the text of the
page. The old method name still works, but issues a DeprecationWarning.
Note that DeprecationWarnings are silent by default, so we might
consider to elevate it in some future version.
Update release notes
Remove the OldPage class

This class provided workarounds for editing pages without 'writeapi'.
Since 'writeapi' has been enabled by default since MediaWiki 1.14,
and we require MediaWiki 1.16, it should no longer be needed. If
'writeapi' has been explicitly disabled, we should rather raise an
error, since this means that automatic edits are not welcome.