update c-lib to upstream

Also update build.rs and ext.c

[Cargo.toml] update rand to 0.6 (breaking change)

Reexport the rand crate

Remove ! from first link, 

Unbreak build on OpenBSD and other OS that aren't linux, windows, or …

The build.rd script tries to find an android toolchain but fails on
other OS, such as the BSDs, that do not provide one.  This change
simply marks them as "unknown" and fails to build with an unknown
android toolchain, but succeeds to build the regular crate on the any
other OS natively.  The fix is not specific to OpenBSD as it unbreaks
all OS that are not linux, windows, or macos.

Remove ! from first link, may be affecting the rendering in superscript in the README in the repo homepage.

* Don't use deprecated rustc-serialize

* Don't depend on libc

Replace `gcc` with a newer `cc` crate

`libsecp` update and some minor enhancements

build secp256 on android

Added methods for secretstore implementation

Add curve points (combine public keys) exposed