make read_preference a public attribute, not a role

This eliminates the _HasReadPreference role.

For MongoClient, read_preference is now a public attribute, with a
somewhat backwards compatible accessor.  For Cursor, read_preference is
a private attribute, but with a public method that overrides it, which
is mostly backwards compatible.
add mailmap, generate contributors section and tweak pod weaving
document alpha status and roadmap
rearrange mongoclient documentation in preparation for heavy editing
add first draft of MongoDB::Upgrading document
reorganize and document connection attributes
Update documentation in preparation for alpha 1 release
make ssl attribute read-only
add topology config attributes to MongoClient
revise MongoDB::Upgrading further
update MongoClient documentation
update connect/disconnect documentation
deprecate auto_connect and make connections always lazy
more tweaks to Upgrading docs
devel: restrict test-all.pl to common configs
devel: heap1 was renamed to inMemoryExperiment