Rename exprlib to library. Name was misleading, it is more than a library for manipulating expressions.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Change lean frontend specific options. Now, frontend specific options must begin with the frontend name.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Update .travis.yml
 - move "Trigger Bitbucket" and "Trigger OSX build" to the
 after_script section so that their failures do not generate build
 failures.
 - move "install doxygen & graphviz" to the after_script to avoid a
 build failure due to the timeout of those package installations.
Add .travis.win.yml for windows build&test