Start accepting [lib], allow `bin = []`

This commit starts to accept `[lib]` as a single library per package. It also
allows opting-out of binaries/tests/examples with a top level `foo = []` rather
than taking an empty array to mean that inference should be enabled.

cc #327
Deprecated [[lib]] in favor of [lib]
auto merge of #369 : alexcrichton/cargo/issue-327, r=wycats

The syntax was originally chosen to perhaps allow multiple libraries in the future, but that is less and less likely to happen at this point. This commit deprecates the now-misleading `[[lib]]` in favor of `[lib]` to indicate that only one can be present.

This does not start allowing `[bin]` or `[example]` and such as I felt that it was too many ways to specify what's essentially the same thing. This also as a bonus allows a top-level `bin = []` to completely opt-out of binary inference (as well as for tests and examples).

Closes #327