- add cargo cache to travis
- fix travis badge into cargo.toml
- 1.17.0 build can fail

Use rust modules to reflect the following code design:
- src/main.rs	cli, setup, logs, IO
- src/lib.rs	core logic (can be re-exported)

The api breaks in different point, particularly there is no more a
`call_main_without_stdin` function.  Because of that the cli arguments
are temporarily dropped.

Structopt is the ergonomic argument-as-structures in rust. The arguments
are organized in commands and subcommands:

- src/main.rs	non-cargo configuration and cli parsing (-v, -q)
- src/lib.rs	real cli parsing and defaults (build [args], etc)

The main returns a Result<(), failure::Error> that wraps all the
possible errors from libraries.

Cargo_metadata is a crate the expose the cargo-metadata subcommand. Rely
on metadata and not on cargo directly reduces considerably both the
runtime and compile-time costs.  The downside is that cargo_metadata
doesn't expose cargo's internal types and metadata. By now is not a
blocking issue because the missing informations can be extracted
directly from the manifest file.  Has been added also another argument
to the `build` to specify a manifest path, because of that now
cargo-ebuild doesn't rely anymore on a cargo project but only on the
manifest.  This change removes also some boilerplate code like resolve,
workspace, etc.

In future work should ported the [package.metadata] field of the
`Cargo.toml` to handle cargo-ebuild specific configurations (es
KEYWORDS).

In future new version of the cargo-metadata format and cargo features
should be mapped into cargo_metadata in order to be used.

The simple test solution is on the ebuild generated for cargo-ebuild
itself. After editing cargo.toml or `cargo update` is necessary to
upadate the test in `test/integration.rs`.