Build common as object library in cmake

Set common as object library to create correct chewing.lib. If we do not set
common as object library, the chewing.lib will not contain functions in
common.lib, and linking to chewing.lib causes unresolved external symbol error.
See [1] for detail information.

[1] http://stackoverflow.com/a/11448878/79816
Manual set output path for Visual Studio

Visual Studio appends configuration (Debug, MinSizeRel, Release,
RelWithDebInfo) into output path, whith is not desirable because we need to
know output path for custom command.